Build Observatory as part of runtime

runtime/observatory/test/weak_properties_test.dart

 // Copyright (c) 2014, 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.
 
 library vm_references_test;
 
 import 'dart:async';
 import 'dart:mirrors';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 44 matching lines @@
       expect(loadedProp.key, isNotNull);
       expect(loadedProp.key.clazz, equals(key.clazz));
       expect(loadedProp.value, isNotNull);
       expect(loadedProp.value.clazz, equals(value.clazz));
     });
   }),
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: script);