Implementation of async-await transformation on js ast.

tests/language/await_regression_test.dart

Index: tests/language/await_regression_test.dart
diff --git a/tests/language/await_regression_test.dart b/tests/language/await_regression_test.dart
index 9c3e663471b03a8c9b8429650c47741ed91bbe41..909764c4a807ce29ea2d93351cedfcef0e0cdcbb 100644
--- a/tests/language/await_regression_test.dart
+++ b/tests/language/await_regression_test.dart
@@ -13,10 +13,10 @@ manana(tequila) async => tequila;
 
 // Regression test for issue 21536.
 testNestedFunctions() async {
-  var a = await later('Asterix').then((tonic) {
+  var a = await (later('Asterix').then((tonic) {
     return later(tonic);
-  });
-  var o = await manana('Obelix').then(manana);
+  }));
+  var o = await (manana('Obelix').then(manana));
   Expect.equals("$a and $o", "Asterix and Obelix");
 }