Infer type of extract index expressions.

R=paulberry@google.com
BUG=

Review URL: https://codereview.chromium.org/1878223002 .

Author: scheglov

pkg/analyzer/lib/src/summary/link.dart

@@ -1791,9 +1791,7 @@ class ExprTypeComputer {
           _doAssignToIndex();
           break;
         case UnlinkedConstOperation.extractIndex:
-          stack.length -= 2;
-          // TODO(paulberry): implement.
-          stack.add(DynamicTypeImpl.instance);
+          _doExtractIndex();
           break;
         case UnlinkedConstOperation.invokeMethodRef:
           _doInvokeMethodRef();
@@ -1924,6 +1922,20 @@ class ExprTypeComputer {
     stack.add(type);
   }
 
+  void _doExtractIndex() {
+    stack.removeLast(); // index
+    DartType target = stack.removeLast();
+    stack.add(() {
+      if (target is InterfaceType) {
+        MethodElement method = target.lookUpMethod('[]', library);
+        if (method != null) {
+          return method.returnType;
+        }
+      }
+      return DynamicTypeImpl.instance;
+    }());
+  }
+
   void _doExtractProperty() {
     DartType target = stack.removeLast();
     String propertyName = _getNextString();

pkg/analyzer/test/src/summary/resynthesize_ast_test.dart

@@ -209,6 +209,33 @@ class AstInferredTypeTest extends AbstractResynthesizeTest
     super.test_genericMethods_inferJSBuiltin();
   }
 
+  void test_infer_extractIndex_custom() {
+    var unit = checkFile('''
+class A {
+  String operator [](_) => null;
+}
+var a = new A();
+var b = a[0];
+  ''');
+    expect(unit.topLevelVariables[1].type.toString(), 'String');
+  }
+
+  void test_infer_extractIndex_fromList() {
+    var unit = checkFile('''
+var a = <int>[1, 2, 3];
+var b = a[0];
+  ''');
+    expect(unit.topLevelVariables[1].type.toString(), 'int');
+  }
+
+  void test_infer_extractIndex_fromMap() {
+    var unit = checkFile('''
+var a = <int, double>{};
+var b = a[0];
+  ''');
+    expect(unit.topLevelVariables[1].type.toString(), 'double');
+  }
+
   void test_infer_extractProperty_getter() {
     checkFile(r'''
 var a = 1.isEven;