Fix dart2js for switches in async code with only awaits in the key.

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library rewrite_async;
 
 import "dart:math" show max;
 import 'dart:collection';
 
 import 'package:_internal/compiler/js_lib/shared/async_await_error_codes.dart'
@@ skipping 2394 matching lines @@
   bool visitReturn(js.Return node) {
     hasExplicitReturns = true;
     targets[node] = currentFunction;
     if (node.value == null) return false;
     return visit(node.value);
   }
 
   @override
   bool visitSwitch(js.Switch node) {
     loopsAndSwitches.add(node);
-    bool result = visit(node.key);
+    // If the key has an `await` expression, do not transform the
+    // body of the switch.
+    visit(node.key);
+    bool result = false;
     for (js.SwitchClause clause in node.cases) {
       if (visit(clause)) result = true;
     }
     loopsAndSwitches.removeLast();
     return result;
   }
 
   @override
   bool visitThis(js.This node) {
     hasThis = true;
@@ skipping 50 matching lines @@
     return condition || body;
   }
 
   @override
   bool visitDartYield(js.DartYield node) {
     hasYield = true;
     visit(node.expression);
     return true;
   }
 }