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

runtime/observatory/tests/service/dominator_tree_test.dart

 // Copyright (c) 2015, 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/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 
 // small example from [Lenguaer & Tarjan 1979]
@@ skipping 39 matching lines @@
   h.x = e; h.y = k;
   i.x = k;
   j.x = i;
   k.x = i; k.y = r;
   l.x = h;
 }
 
 var tests = [
 (Isolate isolate) async {
   var rootLib = await isolate.rootLibrary.load();
-  var snapshot = await isolate.fetchHeapSnapshot().last;
+  var snapshot = await isolate.fetchHeapSnapshot(false).last;
 
   node(String className) {
     var cls = rootLib.classes.singleWhere((cls) => cls.name == className);
     return snapshot.graph.vertices.singleWhere((v) => v.vmCid == cls.vmCid);
   }
 
   expect(node('I').dominator, equals(node('R')));
   expect(node('K').dominator, equals(node('R')));
   expect(node('C').dominator, equals(node('R')));
   expect(node('H').dominator, equals(node('R')));
   expect(node('E').dominator, equals(node('R')));
   expect(node('A').dominator, equals(node('R')));
   expect(node('D').dominator, equals(node('R')));
   expect(node('B').dominator, equals(node('R')));
 
   expect(node('F').dominator, equals(node('C')));
   expect(node('G').dominator, equals(node('C')));
   expect(node('J').dominator, equals(node('G')));
   expect(node('L').dominator, equals(node('D')));
 
   expect(node('R'), isNotNull);  // The field.
 },
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: buildGraph);