Reset _classMembersLengthsIndex in LibraryReader.

This change might help with analyzing the bug.

Bug: #51855
Change-Id: I214a6238e696ffa2ba16df11946e65da18e81945
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/303782
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -496,6 +496,11 @@ class LibraryReader {
     _reader.offset = _offset;
     var resolutionOffset = _baseResolutionOffset + _reader.readUInt30();
 
+    // TODO(scheglov) https://github.com/dart-lang/sdk/issues/51855
+    // This should not be needed.
+    // But I have a suspicion that we attempt to read the library twice.
+    _classMembersLengthsIndex = 0;
+
     var name = _reader.readStringReference();
     var featureSet = _readFeatureSet();