Moved isolate high watermark calculations into the Observatory client.

runtime/observatory/lib/src/service/object.dart

Index: runtime/observatory/lib/src/service/object.dart
diff --git a/runtime/observatory/lib/src/service/object.dart b/runtime/observatory/lib/src/service/object.dart
index ccc9054597b92b533b9fa16d6c53f5a9fdc20d47..74a99ab385916efe704947feba59a7d4f7ebff54 100644
--- a/runtime/observatory/lib/src/service/object.dart
+++ b/runtime/observatory/lib/src/service/object.dart
@@ -1508,7 +1508,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   final List<Thread> _threads = new List<Thread>();
 
   int get memoryHighWatermark => _memoryHighWatermark;
-  int _memoryHighWatermark;
+  int _memoryHighWatermark = 0;
 
   int get numZoneHandles => _numZoneHandles;
   int _numZoneHandles;
@@ -1645,10 +1645,13 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
       threads.addAll(map['_threads']);
     }
 
-    _memoryHighWatermark = int.parse(map['_memoryHighWatermark']);
+    int currentMemoryHighWatermark = 0;
+    for (var i = 0; i < threads.length; i++) {
+      currentMemoryHighWatermark += threads[i].memoryHighWatermark;
+    }
 
-    if (map['threads'] != null) {
-      threads.addAll(map['threads']);
+    if (currentMemoryHighWatermark > _memoryHighWatermark) {
+      _memoryHighWatermark = currentMemoryHighWatermark;
     }
 
     _numZoneHandles = map['_numZoneHandles'];