Add Observatory support for UnlinkedCall, SingleTargetCache and SubtypeTestCache.

runtime/observatory/lib/src/repositories/single_target_cache.dart

+// Copyright (c) 2017, 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 repositories;
+
+class SingleTargetCacheRepository extends M.SingleTargetCacheRepository {
+  Future<M.SingleTargetCache> get(M.IsolateRef i, String id) async {
+    S.Isolate isolate = i as S.Isolate;
+    assert(isolate != null);
+    return (await isolate.getObject(id)) as S.SingleTargetCache;
+  }
+}