Rank most retaining instances for each class.

runtime/observatory/lib/service_common.dart

Index: runtime/observatory/lib/service_common.dart
===================================================================
--- runtime/observatory/lib/service_common.dart	(revision 42649)
+++ runtime/observatory/lib/service_common.dart	(working copy)
@@ -172,7 +172,9 @@
       _webSocket.nonStringToByteData(data).then((ByteData bytes) {
         // See format spec. in VMs Service::SendEvent.
         int offset = 0;
-        int metaSize = bytes.getUint64(offset, Endianness.BIG_ENDIAN);
+        // Dart2JS workaround (no getUint64). Limit to 4 GB metadata.
+        assert(bytes.getUint32(offset, Endianness.BIG_ENDIAN) == 0);
+        int metaSize = bytes.getUint32(offset + 4, Endianness.BIG_ENDIAN);
         offset += 8;
         var meta = _utf8Decoder.convert(new Uint8List.view(
             bytes.buffer, bytes.offsetInBytes + offset, metaSize));