[observatory] Add a 'GC' button to heap map.

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 4c49fcf8df692581aed6045eeb94ada6a49f4fc5..85167d122c90c2257a199324386a837f9b2ebf48 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -3477,6 +3477,18 @@ static const MethodParameter* get_heap_map_params[] = {
 
 static bool GetHeapMap(Thread* thread, JSONStream* js) {
   Isolate* isolate = thread->isolate();
+  bool should_collect = false;
+  if (js->HasParam("gc")) {
+    if (js->ParamIs("gc", "full")) {
+      should_collect = true;
+    } else {
+      PrintInvalidParamError(js, "gc");
+      return true;
+    }
+  }
+  if (should_collect) {
+    isolate->heap()->CollectAllGarbage();
+  }
   isolate->heap()->PrintHeapMapToJSONStream(isolate, js);
   return true;
 }