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

runtime/observatory/lib/src/elements/heap_snapshot.dart

Index: runtime/observatory/lib/src/elements/heap_snapshot.dart
diff --git a/runtime/observatory/lib/src/elements/heap_snapshot.dart b/runtime/observatory/lib/src/elements/heap_snapshot.dart
index a9e4a8ef2d8941889ef18ed861dd2dc95f8e17cc..526ab3372ca3206d31cfe84a43e0395e20e34ed1 100644
--- a/runtime/observatory/lib/src/elements/heap_snapshot.dart
+++ b/runtime/observatory/lib/src/elements/heap_snapshot.dart
@@ -407,8 +407,15 @@ class HeapSnapshotElement extends ObservatoryElement {
     var completer = new Completer();
     state = "Requesting heap snapshot...";
     isolate.getClassRefs();
+
+    var collectGarbage =
+        app.locationManager.uri.queryParameters['collectGarbage'];

[Cutch, 2016/03/17 15:37:32]

Maybe a helper function like bool getBoolParameter(String name, bool
defaultValue);

app.locationManager.getBoolParameter('collectGarbage', true) ?

[rmacnak, 2016/03/17 18:09:08]

Done.

+    if (collectGarbage == null) {
+      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;