Perform a full GC by default when requesting a heap snapshot.

runtime/observatory/tests/service/graph_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.
 // VMOptions=--error_on_bad_type --error_on_bad_override
 
 import 'package:observatory/object_graph.dart';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 
@@ skipping 24 matching lines @@
 int fooId;
 
 var tests = [
 
 (Isolate isolate) async {
   Library lib = await isolate.rootLibrary.load();
   expect(lib.classes.length, equals(1));
   Class fooClass = lib.classes.first;
   fooId = fooClass.vmCid;
 
-  HeapSnapshot snapshot = await isolate.fetchHeapSnapshot().last;
+  HeapSnapshot snapshot = await isolate.fetchHeapSnapshot(false).last;
   ObjectGraph graph = snapshot.graph;
 
   expect(fooId, isNotNull);
   Iterable<ObjectVertex> foos = graph.vertices.where(
       (ObjectVertex obj) => obj.vmCid == fooId);
   expect(foos.length, equals(3));
   expect(foos.where((obj) => obj.successors.length == 0).length,
          equals(1));
   expect(foos.where((obj) => obj.successors.length == 1).length,
          equals(1));
@@ skipping 33 matching lines @@
   expect(first.successors.length,
          equals(2 + second.successors.length));
   // ... and specifically, that it retains exactly itself + the long one.
   expect(first.retainedSize,
          equals(first.shallowSize + second.shallowSize));
 },
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: script);