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

runtime/observatory/lib/src/elements/heap_snapshot.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.
 
 library heap_snapshot_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'observatory_element.dart';
 import 'package:observatory/app.dart';
@@ skipping 389 matching lines @@
   }
 
   Future refresh() {
     return _getHeapSnapshot();
   }
 
   Future _getHeapSnapshot() {
     var completer = new Completer();
     state = "Requesting heap snapshot...";
     isolate.getClassRefs();
+
+    bool collectGarbage =
+        app.locationManager.getBoolParameter('collectGarbage', true);
+
     var stopwatch = new Stopwatch()..start();
-    isolate.fetchHeapSnapshot().listen((event) {
+    isolate.fetchHeapSnapshot(collectGarbage).listen((event) {
       if (event is String) {
         print("${stopwatch.elapsedMilliseconds} $event");
         state = event;
       } else if (event is HeapSnapshot) {
         snapshot = event;
         state = 'Loaded';
         completer.complete(snapshot);
         _update();
       } else {
         throw "Unexpected event $event";
@@ skipping 34 matching lines @@
 
     new Future.delayed(const Duration(milliseconds: 500), () {
       buildMergedVertices(snapshot.graph).then((vertices) {
         state = 'Loaded';
         var rootRow = new MergedVerticesRow(tree, null, isolate, vertices);
         tree.initialize(rootRow);
       });
     });
   }
 }