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

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index f305c766860680695a568aabd6776f756ab76de4..22a0707b418381284c886d1caf05c0ae7cdd6b18 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -827,6 +827,11 @@ class CodeGenerator extends tree_ir.StatementVisitor
     }
   }
 
+  @override
+  js.Expression visitAwait(tree_ir.Await node) {
+    return new js.Await(visitExpression(node.input));
+  }
+
   visitFunctionExpression(tree_ir.FunctionExpression node) {
     // FunctionExpressions are currently unused.
     // We might need them if we want to emit raw JS nested functions.