Added isolate + thread high watermark tracking to Observatory

runtime/observatory/tests/service/get_zone_memory_info_rpc_test.dart

Index: runtime/observatory/tests/service/get_zone_memory_info_rpc_test.dart
diff --git a/runtime/observatory/tests/service/get_zone_memory_info_rpc_test.dart b/runtime/observatory/tests/service/get_zone_memory_info_rpc_test.dart
index 7d5f00452757b0fa556c6cc51ccffae1d2942b4f..36876becc0ca0bf55d535d54be9ab852f22d2a39 100644
--- a/runtime/observatory/tests/service/get_zone_memory_info_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_zone_memory_info_rpc_test.dart
@@ -15,7 +15,7 @@ var tests = [
     // the correct fields needed to examine zone memory usage.
     for (Isolate isolate in vm.isolates) {
       await isolate.reload();
-
+      expect(isolate.memoryHighWatermark, isInt);
       expect(isolate.threads, isNotNull);
       List<Thread> threads = isolate.threads;
 
@@ -23,6 +23,7 @@ var tests = [
         expect(thread.type, equals('_Thread'));
         expect(thread.id, isNotNull);
         expect(thread.kind, isNotNull);
+        expect(thread.memoryHighWatermark, isInt);
         expect(thread.zones, isNotNull);
         List<Zone> zones = thread.zones;