diff --git a/core/src/main/java/com/google/common/truth/IgnoreJRERequirement.java b/core/src/main/java/com/google/common/truth/IgnoreJRERequirement.java
new file mode 100644
index 0000000000..a2d4842727
--- /dev/null
+++ b/core/src/main/java/com/google/common/truth/IgnoreJRERequirement.java
@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2019 Google, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.common.truth;
+
+import static java.lang.annotation.ElementType.CONSTRUCTOR;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.ElementType.TYPE;
+
+import java.lang.annotation.Target;
+
+/**
+ * Disables Animal Sniffer's checking of compatibility with older versions of Java/Android.
+ */
+@Target({METHOD, CONSTRUCTOR, TYPE})
+@interface IgnoreJRERequirement {}
diff --git a/extensions/java8/src/main/java/com/google/common/truth/IntStreamSubject.java b/core/src/main/java/com/google/common/truth/IntStreamSubject.java
similarity index 96%
rename from extensions/java8/src/main/java/com/google/common/truth/IntStreamSubject.java
rename to core/src/main/java/com/google/common/truth/IntStreamSubject.java
index 45699bce8b..95270b61fa 100644
--- a/extensions/java8/src/main/java/com/google/common/truth/IntStreamSubject.java
+++ b/core/src/main/java/com/google/common/truth/IntStreamSubject.java
@@ -40,12 +40,16 @@
  *
  * @author Kurt Alfred Kluever
  */
-@SuppressWarnings("deprecation") // TODO(b/134064106): design an alternative to no-arg check()
+@SuppressWarnings({
+  "deprecation", // TODO(b/134064106): design an alternative to no-arg check()
+  "Java7ApiChecker", // used only from APIs with Java 8 in their signatures
+})
+@IgnoreJRERequirement
 public final class IntStreamSubject extends Subject {
 
   private final List<?> actualList;
 
-  private IntStreamSubject(FailureMetadata failureMetadata, @Nullable IntStream stream) {
+  IntStreamSubject(FailureMetadata failureMetadata, @Nullable IntStream stream) {
     super(failureMetadata, stream);
     this.actualList =
         (stream == null) ? null : stream.boxed().collect(toCollection(ArrayList::new));
diff --git a/extensions/java8/src/main/java/com/google/common/truth/LongStreamSubject.java b/core/src/main/java/com/google/common/truth/LongStreamSubject.java
similarity index 96%
rename from extensions/java8/src/main/java/com/google/common/truth/LongStreamSubject.java
rename to core/src/main/java/com/google/common/truth/LongStreamSubject.java
index 0a5c22557c..70213a5cd8 100644
--- a/extensions/java8/src/main/java/com/google/common/truth/LongStreamSubject.java
+++ b/core/src/main/java/com/google/common/truth/LongStreamSubject.java
@@ -40,12 +40,16 @@
  *
  * @author Kurt Alfred Kluever
  */
-@SuppressWarnings("deprecation") // TODO(b/134064106): design an alternative to no-arg check()
+@SuppressWarnings({
+  "deprecation", // TODO(b/134064106): design an alternative to no-arg check()
+  "Java7ApiChecker", // used only from APIs with Java 8 in their signatures
+})
+@IgnoreJRERequirement
 public final class LongStreamSubject extends Subject {
 
   private final List<?> actualList;
 
-  private LongStreamSubject(FailureMetadata failureMetadata, @Nullable LongStream stream) {
+  LongStreamSubject(FailureMetadata failureMetadata, @Nullable LongStream stream) {
     super(failureMetadata, stream);
     this.actualList =
         (stream == null) ? null : stream.boxed().collect(toCollection(ArrayList::new));
diff --git a/extensions/java8/src/main/java/com/google/common/truth/OptionalDoubleSubject.java b/core/src/main/java/com/google/common/truth/OptionalDoubleSubject.java
similarity index 96%
rename from extensions/java8/src/main/java/com/google/common/truth/OptionalDoubleSubject.java
rename to core/src/main/java/com/google/common/truth/OptionalDoubleSubject.java
index 4a9aa5f88b..7265fcac8a 100644
--- a/extensions/java8/src/main/java/com/google/common/truth/OptionalDoubleSubject.java
+++ b/core/src/main/java/com/google/common/truth/OptionalDoubleSubject.java
@@ -26,6 +26,8 @@
  *
  * @author Ben Douglass
  */
+@SuppressWarnings("Java7ApiChecker") // used only from APIs with Java 8 in their signatures
+@IgnoreJRERequirement
 public final class OptionalDoubleSubject extends Subject {
 
   private final OptionalDouble actual;
diff --git a/extensions/java8/src/main/java/com/google/common/truth/OptionalIntSubject.java b/core/src/main/java/com/google/common/truth/OptionalIntSubject.java
similarity index 95%
rename from extensions/java8/src/main/java/com/google/common/truth/OptionalIntSubject.java
rename to core/src/main/java/com/google/common/truth/OptionalIntSubject.java
index 957594f720..cafdd5aff0 100644
--- a/extensions/java8/src/main/java/com/google/common/truth/OptionalIntSubject.java
+++ b/core/src/main/java/com/google/common/truth/OptionalIntSubject.java
@@ -26,6 +26,8 @@
  *
  * @author Ben Douglass
  */
+@SuppressWarnings("Java7ApiChecker") // used only from APIs with Java 8 in their signatures
+@IgnoreJRERequirement
 public final class OptionalIntSubject extends Subject {
   private final OptionalInt actual;
 
diff --git a/extensions/java8/src/main/java/com/google/common/truth/OptionalLongSubject.java b/core/src/main/java/com/google/common/truth/OptionalLongSubject.java
similarity index 95%
rename from extensions/java8/src/main/java/com/google/common/truth/OptionalLongSubject.java
rename to core/src/main/java/com/google/common/truth/OptionalLongSubject.java
index 9b57b9a4a9..ab770a3237 100644
--- a/extensions/java8/src/main/java/com/google/common/truth/OptionalLongSubject.java
+++ b/core/src/main/java/com/google/common/truth/OptionalLongSubject.java
@@ -26,6 +26,8 @@
  *
  * @author Ben Douglass
  */
+@SuppressWarnings("Java7ApiChecker") // used only from APIs with Java 8 in their signatures
+@IgnoreJRERequirement
 public final class OptionalLongSubject extends Subject {
   private final OptionalLong actual;
 
diff --git a/extensions/java8/src/main/java/com/google/common/truth/OptionalSubject.java b/core/src/main/java/com/google/common/truth/OptionalSubject.java
similarity index 96%
rename from extensions/java8/src/main/java/com/google/common/truth/OptionalSubject.java
rename to core/src/main/java/com/google/common/truth/OptionalSubject.java
index 65ca4dae95..8fa39adffa 100644
--- a/extensions/java8/src/main/java/com/google/common/truth/OptionalSubject.java
+++ b/core/src/main/java/com/google/common/truth/OptionalSubject.java
@@ -26,6 +26,8 @@
  *
  * @author Christian Gruber
  */
+@SuppressWarnings("Java7ApiChecker") // used only from APIs with Java 8 in their signatures
+@IgnoreJRERequirement
 public final class OptionalSubject extends Subject {
   private final @Nullable Optional<?> actual;
 
diff --git a/extensions/java8/src/main/java/com/google/common/truth/PathSubject.java b/core/src/main/java/com/google/common/truth/PathSubject.java
similarity index 92%
rename from extensions/java8/src/main/java/com/google/common/truth/PathSubject.java
rename to core/src/main/java/com/google/common/truth/PathSubject.java
index 0be8532e05..f98c60093e 100644
--- a/extensions/java8/src/main/java/com/google/common/truth/PathSubject.java
+++ b/core/src/main/java/com/google/common/truth/PathSubject.java
@@ -22,8 +22,9 @@
 /** Assertions for {@link Path} instances. */
 @GwtIncompatible
 @J2ObjCIncompatible
+@J2ktIncompatible
 public final class PathSubject extends Subject {
-  private PathSubject(FailureMetadata failureMetadata, Path actual) {
+  PathSubject(FailureMetadata failureMetadata, Path actual) {
     super(failureMetadata, actual);
   }
 
diff --git a/core/src/main/java/com/google/common/truth/StandardSubjectBuilder.java b/core/src/main/java/com/google/common/truth/StandardSubjectBuilder.java
index f41fb486f3..0d90808b75 100644
--- a/core/src/main/java/com/google/common/truth/StandardSubjectBuilder.java
+++ b/core/src/main/java/com/google/common/truth/StandardSubjectBuilder.java
@@ -18,7 +18,6 @@
 import static com.google.common.base.Preconditions.checkNotNull;
 
 import com.google.common.annotations.GwtIncompatible;
-import com.google.common.base.Optional;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Multimap;
 import com.google.common.collect.Multiset;
@@ -149,7 +148,7 @@ public final PrimitiveDoubleArraySubject that(double @Nullable [] actual) {
     return new PrimitiveDoubleArraySubject(metadata(), actual, "array");
   }
 
-  public final GuavaOptionalSubject that(@Nullable Optional<?> actual) {
+  public final GuavaOptionalSubject that(com.google.common.base.@Nullable Optional<?> actual) {
     return new GuavaOptionalSubject(metadata(), actual, "optional");
   }
 
diff --git a/extensions/java8/src/main/java/com/google/common/truth/StreamSubject.java b/core/src/main/java/com/google/common/truth/StreamSubject.java
similarity index 97%
rename from extensions/java8/src/main/java/com/google/common/truth/StreamSubject.java
rename to core/src/main/java/com/google/common/truth/StreamSubject.java
index f3e90cd6e4..45ed8f5f21 100644
--- a/extensions/java8/src/main/java/com/google/common/truth/StreamSubject.java
+++ b/core/src/main/java/com/google/common/truth/StreamSubject.java
@@ -40,12 +40,16 @@
  *
  * @author Kurt Alfred Kluever
  */
-@SuppressWarnings("deprecation") // TODO(b/134064106): design an alternative to no-arg check()
+@SuppressWarnings({
+  "deprecation", // TODO(b/134064106): design an alternative to no-arg check()
+  "Java7ApiChecker", // used only from APIs with Java 8 in their signatures
+})
+@IgnoreJRERequirement
 public final class StreamSubject extends Subject {
 
   private final List<?> actualList;
 
-  private StreamSubject(FailureMetadata failureMetadata, @Nullable Stream<?> stream) {
+  StreamSubject(FailureMetadata failureMetadata, @Nullable Stream<?> stream) {
     super(failureMetadata, stream);
     this.actualList = (stream == null) ? null : stream.collect(toCollection(ArrayList::new));
   }
diff --git a/core/src/main/java/com/google/common/truth/Truth.java b/core/src/main/java/com/google/common/truth/Truth.java
index 1afbccaa17..6cf4ca7e89 100644
--- a/core/src/main/java/com/google/common/truth/Truth.java
+++ b/core/src/main/java/com/google/common/truth/Truth.java
@@ -18,7 +18,6 @@
 import static com.google.common.base.Preconditions.checkNotNull;
 
 import com.google.common.annotations.GwtIncompatible;
-import com.google.common.base.Optional;
 import com.google.common.collect.Multimap;
 import com.google.common.collect.Multiset;
 import com.google.common.collect.Table;
@@ -229,7 +228,8 @@ public static PrimitiveDoubleArraySubject assertThat(double @Nullable [] actual)
     return assert_().that(actual);
   }
 
-  public static GuavaOptionalSubject assertThat(@Nullable Optional<?> actual) {
+  public static GuavaOptionalSubject assertThat(
+      com.google.common.base.@Nullable Optional<?> actual) {
     return assert_().that(actual);
   }
 
diff --git a/extensions/java8/src/main/java/com/google/common/truth/Truth8.java b/core/src/main/java/com/google/common/truth/Truth8.java
similarity index 95%
rename from extensions/java8/src/main/java/com/google/common/truth/Truth8.java
rename to core/src/main/java/com/google/common/truth/Truth8.java
index c156c54b86..c34a975130 100644
--- a/extensions/java8/src/main/java/com/google/common/truth/Truth8.java
+++ b/core/src/main/java/com/google/common/truth/Truth8.java
@@ -43,6 +43,8 @@
  * <a href="https://truth.dev/faq#full-chain">How do I specify a custom message/failure
  * behavior/{@code Subject} type?</a> in the Truth FAQ.
  */
+// The methods here are no more dangerous that wherever the user got the (e.g.) Stream.
+@SuppressWarnings("Java7ApiChecker")
 public final class Truth8 {
   @SuppressWarnings("AssertAboutOptionals") // suggests infinite recursion
   public static OptionalSubject assertThat(@Nullable Optional<?> target) {
@@ -79,6 +81,7 @@ public static LongStreamSubject assertThat(@Nullable LongStream target) {
   // to support Java environments without java.nio.file such as Android and J2CL.
   @GwtIncompatible
   @J2ObjCIncompatible
+  @J2ktIncompatible
   public static PathSubject assertThat(@Nullable Path target) {
     return assertAbout(PathSubject.paths()).that(target);
   }
diff --git a/core/src/test/java/com/google/common/truth/gwt/Inventory.java b/core/src/test/java/com/google/common/truth/gwt/Inventory.java
index 63e2fbcc1f..96ce5e1ebe 100644
--- a/core/src/test/java/com/google/common/truth/gwt/Inventory.java
+++ b/core/src/test/java/com/google/common/truth/gwt/Inventory.java
@@ -23,13 +23,19 @@
 import com.google.common.truth.FailureStrategy;
 import com.google.common.truth.FloatSubject;
 import com.google.common.truth.GuavaOptionalSubject;
+import com.google.common.truth.IntStreamSubject;
 import com.google.common.truth.IntegerSubject;
 import com.google.common.truth.IterableSubject;
+import com.google.common.truth.LongStreamSubject;
 import com.google.common.truth.LongSubject;
 import com.google.common.truth.MapSubject;
 import com.google.common.truth.MultimapSubject;
 import com.google.common.truth.MultisetSubject;
 import com.google.common.truth.ObjectArraySubject;
+import com.google.common.truth.OptionalDoubleSubject;
+import com.google.common.truth.OptionalIntSubject;
+import com.google.common.truth.OptionalLongSubject;
+import com.google.common.truth.OptionalSubject;
 import com.google.common.truth.Ordered;
 import com.google.common.truth.PrimitiveBooleanArraySubject;
 import com.google.common.truth.PrimitiveByteArraySubject;
@@ -39,11 +45,13 @@
 import com.google.common.truth.PrimitiveIntArraySubject;
 import com.google.common.truth.PrimitiveLongArraySubject;
 import com.google.common.truth.PrimitiveShortArraySubject;
+import com.google.common.truth.StreamSubject;
 import com.google.common.truth.StringSubject;
 import com.google.common.truth.Subject;
 import com.google.common.truth.TableSubject;
 import com.google.common.truth.ThrowableSubject;
 import com.google.common.truth.Truth;
+import com.google.common.truth.Truth8;
 import com.google.common.truth.TruthJUnit;
 
 /**
@@ -59,12 +67,18 @@ public class Inventory {
   FloatSubject floatSubject;
   GuavaOptionalSubject guavaOptionalSubject;
   IntegerSubject integerSubject;
+  IntStreamSubject intStreamSubject;
   IterableSubject iterableSubject;
   LongSubject longSubject;
+  LongStreamSubject longStreamSubject;
   MapSubject mapSubject;
   MultimapSubject multimapSubject;
   MultisetSubject multisetSubject;
   ObjectArraySubject<?> objectArraySubject;
+  OptionalSubject optionalSubject;
+  OptionalDoubleSubject optionalDoubleSubject;
+  OptionalIntSubject optionalIntSubject;
+  OptionalLongSubject optionalLongSubject;
   Ordered ordered;
   PrimitiveBooleanArraySubject primitiveBooleanArraySubject;
   PrimitiveByteArraySubject primitiveByteArraySubject;
@@ -74,11 +88,13 @@ public class Inventory {
   PrimitiveIntArraySubject primitiveIntArraySubject;
   PrimitiveLongArraySubject primitiveLongArraySubject;
   PrimitiveShortArraySubject primitiveShortArraySubject;
+  StreamSubject streamSubject;
   StringSubject stringSubject;
   Subject.Factory<?, ?> subjectFactory;
   Subject subject;
   TableSubject tableSubject;
   ThrowableSubject throwableSubject;
   Truth truth;
+  Truth8 truth8;
   TruthJUnit truthJUnit;
 }
diff --git a/core/src/test/java/com/google/common/truth/gwt/TruthTest.gwt.xml b/core/src/test/java/com/google/common/truth/gwt/TruthTest.gwt.xml
index ee9caccb99..a0362fbd8b 100644
--- a/core/src/test/java/com/google/common/truth/gwt/TruthTest.gwt.xml
+++ b/core/src/test/java/com/google/common/truth/gwt/TruthTest.gwt.xml
@@ -3,6 +3,5 @@
   <inherits name="com.google.common.collect.Collect"/>
   <inherits name="com.google.common.primitives.Primitives"/>
   <inherits name="com.google.common.truth.Truth"/>
-  <!-- TODO(cpovirk): Test Truth8. -->
   <inherits name="com.google.gwt.junit.JUnit"/>
 </module>
diff --git a/extensions/java8/src/main/java/com/google/common/truth/Truth8.gwt.xml b/extensions/java8/src/main/java/com/google/common/truth/Truth8.gwt.xml
deleted file mode 100644
index f9aa5adff7..0000000000
--- a/extensions/java8/src/main/java/com/google/common/truth/Truth8.gwt.xml
+++ /dev/null
@@ -1,35 +0,0 @@
-<module>
-<source path="">
-  <!-- Hack to keep collect from hiding collect.testing supersource: -->
-  <exclude name="**/testing/**"/>
-</source>
-
-<!--
-    We used to set this only for packages that had manual supersource. That
-    worked everywhere that I know of except for one place: when running the GWT
-    util.concurrent tests under Guava.
-
-    The problem is that GWT responds poorly to two .gwt.xml files in the same
-    Java package; see https://goo.gl/pRV3Yn for details.
-
-    The summary is that it ignores one file in favor of the other.
-    util.concurrent, like nearly all our packages, has two .gwt.xml files: one
-    for prod and one for tests. However, unlike our other packages, as of this
-    writing it has test supersource but no prod supersource.
-
-    GWT happens to use the prod .gwt.xml, so it looks for no supersource for
-    tests, either. This causes it to fail to find AtomicLongMapTest.
-
-    Our workaround is to tell GWT that util.concurrent and all other packages
-    have prod supersource, even if they have none. GWT is happy to ignore us
-    when we specify a nonexistent path.
-
-    (I hope that this workaround does not cause its own problems in the future.)
--->
-<super-source path="super"/>
-
-<inherits name="com.google.common.annotations.Annotations" />
-<inherits name="com.google.common.truth.Truth" />
-<inherits name="com.google.gwt.core.Core" />
-<inherits name="com.google.gwt.user.User" />
-</module>
diff --git a/pom.xml b/pom.xml
index 0962f1d433..d66034df07 100644
--- a/pom.xml
+++ b/pom.xml
@@ -263,6 +263,7 @@
           <artifactId>animal-sniffer-maven-plugin</artifactId>
           <version>1.23</version>
           <configuration>
+            <annotations>com.google.common.truth.IgnoreJRERequirement</annotations>
             <signature>
               <groupId>com.toasttab.android</groupId>
               <artifactId>gummy-bears-api-19</artifactId>