[vm service] Collect garbage before getting native allocation samples

runtime/observatory/tests/service/collect_all_garbage_test.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.
+// VMOptions=--error_on_bad_type --error_on_bad_override
+
+import 'dart:async';
+import 'package:observatory/service_io.dart';
+import 'package:unittest/unittest.dart';
+
+import 'test_helper.dart';
+
+var tests = [
+  (Isolate isolate) async {
+    var result = await isolate.invokeRpcNoUpgrade('_collectAllGarbage', {});
+    expect(result['type'], equals('Success'));
+  },
+];
+
+main(args) async => runIsolateTests(args, tests);