Added isolate + thread high watermark tracking to Observatory

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of models;
 
 enum ThreadKind {
   unknownTask,
   mutatorTask,
   compilerTask,
   sweeperTask,
   markerTask,
   finalizerTask
 }
 
 abstract class Thread {
   /// The id associated with the thread on creation.
   String get id;
 
   /// 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;
+
   /// A list of all the zones held by the thread.
   Iterable<Zone> get zones;
 }