Use a VariableUse instead of VariableDeclaration for left-hand side of

pkg/compiler/lib/src/js/rewrite_async.dart

Index: pkg/compiler/lib/src/js/rewrite_async.dart
diff --git a/pkg/compiler/lib/src/js/rewrite_async.dart b/pkg/compiler/lib/src/js/rewrite_async.dart
index 54f5bbed9faf0883967d314b2f0dd8063c873457..cc294708ad77e6929650b83042667d8deb96bc4b 100644
--- a/pkg/compiler/lib/src/js/rewrite_async.dart
+++ b/pkg/compiler/lib/src/js/rewrite_async.dart
@@ -1613,7 +1613,7 @@ class AsyncRewriter extends js.NodeVisitor {
       if (initialization.value != null) {
         withExpression(initialization.value, (js.Expression value) {
           addStatement(new js.ExpressionStatement(
-              new js.Assignment(declaration, value)));
+              new js.Assignment(new js.VariableUse(declaration.name), value)));
         }, store: false);
       }
     }