Add VMService GC events

runtime/bin/vmservice/observatory/test/gc_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 gc_test;
+
+import 'test_helper.dart';
+import 'package:observatory/service_io.dart';
+import 'package:unittest/unittest.dart';
+
+import 'dart:io';
+
+// Expect at least this many GC events.
+int gcCountdown = 3;
+
+void onEvent(ServiceEvent event) {
+  if (event.eventType != 'GC') {
+    return;
+  }
+  if (--gcCountdown == 0) {
+    exit(0);
+  }
+}
+
+main() {
+  String script = 'gc_script.dart';
+  var process = new TestLauncher(script);
+  process.launch().then((port) {
+    String addr = 'ws://localhost:$port/ws';
+    new WebSocketVM(new WebSocketVMTarget(addr)).get('vm')
+        .then((VM vm) {
+      vm.events.stream.listen(onEvent);
+    });
+  });
+}