Run async immediately in dart2js and VM.

runtime/lib/async_patch.dart

Index: runtime/lib/async_patch.dart
diff --git a/runtime/lib/async_patch.dart b/runtime/lib/async_patch.dart
index 7186eb094c12622af2c48814a34f6a46a5c23701..cbd16be51062f999cb348b75b37bbb8dec5afbd0 100644
--- a/runtime/lib/async_patch.dart
+++ b/runtime/lib/async_patch.dart
@@ -7,6 +7,41 @@ import "dart:_internal" hide Symbol;
 // Equivalent of calling FATAL from C++ code.
 _fatal(msg) native "DartAsync_fatal";
 
+class _AsyncAwaitCompleter<T> implements Completer<T> {
+  final _completer = new Completer<T>.sync();
+  bool isSync;
+
+  _AsyncAwaitCompleter() : isSync = false;
+
+  void complete([T value]) {
+    if (isSync) {
+      _completer.complete(value);
+    } else {
+      scheduleMicrotask(() {
+        _completer.complete(value);
+      });
+    }
+  }
+
+  void completeError(e, [st]) {
+    if (isSync) {
+      _completer.completeError(e, st);
+    } else {
+      scheduleMicrotask(() {
+        _completer.completeError(e, st);
+      });
+    }
+  }
+
+  void start(f) {
+    f();
+    isSync = true;
+  }
+
+  Future<T> get future => _completer.future;
+  bool get isCompleted => _completer.isCompleted;
+}
+
 // We need to pass the value as first argument and leave the second and third
 // arguments empty (used for error handling).
 // See vm/ast_transformer.cc for usage.