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

runtime/observatory/lib/src/service/object.dart

Index: runtime/observatory/lib/src/service/object.dart
diff --git a/runtime/observatory/lib/src/service/object.dart b/runtime/observatory/lib/src/service/object.dart
index 19a086c4519e48c9eedbfe179345802fecb5fd2c..4fe8aadb885e325e0f8ee913338d665db1664369 100644
--- a/runtime/observatory/lib/src/service/object.dart
+++ b/runtime/observatory/lib/src/service/object.dart
@@ -1365,11 +1365,12 @@ class Isolate extends ServiceObjectOwner {
     }
   }
 
-  Stream fetchHeapSnapshot() {
+  Stream fetchHeapSnapshot(collectGarbage) {
     if (_snapshotFetch == null || _snapshotFetch.isClosed) {
       _snapshotFetch = new StreamController();
       // isolate.vm.streamListen('_Graph');
-      isolate.invokeRpcNoUpgrade('_requestHeapSnapshot', {});
+      isolate.invokeRpcNoUpgrade('_requestHeapSnapshot',
+                                 {'collectGarbage': collectGarbage});
     }
     return _snapshotFetch.stream;
   }