dart2js cps: Support async/await by rewriting the JavaScript AST.

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
index 6c864be8ee7c1205e3a6a0ae9920635871c07509..399b972928ecda0a83224c962eb0ffe009b31ef2 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
@@ -1107,6 +1107,11 @@ class StatementRewriter extends Transformer implements Pass {
     _rewriteList(node.arguments);
     return node;
   }
+
+  @override
+  Expression visitAwait(Await node) {
+    return node;
+  }
 }
 
 /// Result of combining two expressions, with the potential for reverting the