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

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;
+
+  /// A list of all the zones held by the thread.
+  Iterable<Zone> get zones;
+}