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

runtime/observatory/lib/src/models/objects/thread.dart

Index: runtime/observatory/lib/src/models/objects/thread.dart
diff --git a/runtime/observatory/lib/src/models/objects/thread.dart b/runtime/observatory/lib/src/models/objects/thread.dart
index 7bff72c9191eb98a34a8a0a5a15c7741dee29909..df1af3a058fc22abaabf9c5f18df2511c00f0537 100644
--- a/runtime/observatory/lib/src/models/objects/thread.dart
+++ b/runtime/observatory/lib/src/models/objects/thread.dart
@@ -20,10 +20,13 @@ abstract class Thread {
   /// The task type associated with the thread.
   ThreadKind get kind;
 
-  /// The maximum amount of memory in bytes allocated by a thread at a given
-  /// time throughout the entire life of the thread.
-  int get memoryHighWatermark;
+  /// The maximum amount of zone memory in bytes allocated by a thread at a
+  /// given time throughout the entire life of the thread.
+  int get zoneHighWatermark;
 
-  /// A list of all the zones held by the thread.
-  Iterable<Zone> get zones;
+  /// The current Zone capacity available to this thread.
+  int get zoneCapacity;
+
+  /// The current Zone usage by this thread.
+  int get zoneUsage;
 }