Reimplemented zone memory tracking to avoid race conditions that were causing crashes in the previo…

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 36876becc0ca0bf55d535d54be9ab852f22d2a39..3da96077f523bed50b143e0423d32bb085dc2f05 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.zoneHighWatermark, isInt);
       expect(isolate.threads, isNotNull);
       List<Thread> threads = isolate.threads;
 
@@ -23,14 +23,8 @@ 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;
-
-        for (Zone zone in zones) {
-          expect(zone.capacity, isInt);
-          expect(zone.used, isInt);
-        }
+        expect(thread.zoneHighWatermark, isInt);
+        expect(thread.zoneCapacity, isInt);
       }
     }
   },