Run async immediately in dart2js and VM.

sdk/lib/_internal/js_runtime/lib/async_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/async_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/async_patch.dart b/sdk/lib/_internal/js_runtime/lib/async_patch.dart
index ce7b0d1e838846313c059124ede83db951e4db02..6e96ac48d7bbf7e90754830d93feefc6de50a561 100644
--- a/sdk/lib/_internal/js_runtime/lib/async_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/async_patch.dart
@@ -130,6 +130,36 @@ class Timer {
   }
 }
 
+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);
+      });
+    }
+  }
+
+  Future<T> get future => _completer.future;
+  bool get isCompleted => _completer.isCompleted;
+}
+
 /// Initiates the computation of an `async` function.
 ///
 /// Used as part of the runtime support for the async/await transformation.
@@ -137,9 +167,14 @@ class Timer {
 /// This function sets up the first call into the transformed [bodyFunction].
 /// Independently, it takes the [completer] and returns the future of the
 /// completer for convenience of the transformed code.
-dynamic _asyncStart(_WrappedAsyncBody bodyFunction, Completer completer) {
-  // TODO(sra): Specialize this implementation of `await null`.
-  _awaitOnObject(null, bodyFunction);
+dynamic _asyncStart(
+    _WrappedAsyncBody bodyFunction, _AsyncAwaitCompleter completer) {
+  if (completer.isSync) {
+    // Can't happen. Just here to make the class known to the compiler.

[sra1, 2017/08/24 17:21:18]

This should be done properly through the impact system.

+    completer = new _AsyncAwaitCompleter();
+  }
+  bodyFunction(async_error_codes.SUCCESS, null);
+  completer.isSync = true;
   return completer.future;
 }