[analyzer] Slimmer _StringTable

_StringTable had 2 Uint32Lists for offsets and lengths in order to be
able to read strings lazily.
As offsets[x+1] = offsets[x]+length[x] we don't really need the lengths
one though.

Analyzing (82 contexts in) flutter/flutter (without a cache) this
reduces memory load by something like 22-27 MB (see below).

Note that it doesn't actually reduce the heap capacity or the process
RSS in this instance though.

Statistics on 5 runs of each:

```
_Uint32List (dart:typed_data) (bytes):
Difference at 95.0% confidence
   -23842069.33 +/- 986160.19
   -5.15% +/- 0.21%

heapUsage:
Difference at 95.0% confidence
   -28619536.00 +/- 8755493.42
   -0.79% +/- 0.24%
```

Change-Id: Ia71bac2233fcef2d9e930c8b78f1aee58e1af78f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/320801
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

Author: jensjoha

pkg/analyzer/lib/src/summary2/data_reader.dart

@@ -195,7 +195,6 @@ class _StringTable {
   int _byteOffset;
 
   late final Uint32List _offsets;
-  late final Uint32List _lengths;
   late final List<String?> _strings;
 
   /// The structure of the table:
@@ -211,14 +210,13 @@ class _StringTable {
     var offset = startOffset - _readUInt30();
     var length = _readUInt30();
 
-    _offsets = Uint32List(length);
-    _lengths = Uint32List(length);
+    _offsets = Uint32List(length + 1);
     for (var i = 0; i < length; i++) {
       var stringLength = _readUInt30();
       _offsets[i] = offset;
-      _lengths[i] = stringLength;
       offset += stringLength;
     }
+    _offsets[length] = offset;
 
     _strings = List.filled(length, null, growable: false);
   }
@@ -227,7 +225,10 @@ class _StringTable {
     var result = _strings[index];
 
     if (result == null) {
-      result = _readStringEntry(_offsets[index], _lengths[index]);
+      int start = _offsets[index];
+      int end = _offsets[index + 1];
+      int length = end - start;
+      result = _readStringEntry(_offsets[index], length);
       result = considerCanonicalizeString(result);
       _strings[index] = result;
     }