Added thread.dart and zone.dart to the observatory sources to resolve build issues.

runtime/observatory/lib/src/models/objects/isolate.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;
 
 abstract class IsolateRef {
   /// The id which is passed to the getIsolate RPC to reload this
   /// isolate.
   String get id;
@@ skipping 33 matching lines @@
   ///
   /// Guaranteed to be initialized when the IsolateRunnable event fires.
   Iterable<LibraryRef> get libraries;
 
   /// A list of all breakpoints for this isolate.
   //Iterable<Breakpoint> get breakpoints;
 
   /// [optional] The error that is causing this isolate to exit, if applicable.
   Error get error;
 
+  /// The list of threads associated with this isolate.
+  Iterable<Thread> get threads;
+
   /// The current pause on exception mode for this isolate.
   //ExceptionPauseMode get exceptionPauseMode;
 
   /// [optional] The list of service extension RPCs that are registered for this
   /// isolate, if any.
   Iterable<String> get extensionRPCs;
 
   Map get counters;
   HeapSpace get newSpace;
   HeapSpace get oldSpace;
 
   IsolateStatus get status;
 
   /// [optional]
   FunctionRef get entry;
 }