Initialize "nextWhenCanceled" variable in async rewrite.

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 830 matching lines @@
       inits.add(makeInit(completerName, new js.New(newCompleter, [])));
     } else if (isAsyncStar) {
       inits.add(makeInit(controllerName,
           new js.Call(newController, [new js.VariableUse(bodyName)])));
     }
     inits.add(makeInit(handlerName, js.number(rethrowLabel)));
     inits.add(makeInit(currentErrorName, null));
     if (hasJumpThroughFinally || analysis.hasYield) {
       inits.add(makeInit(nextName, null));
     }
+    if (isAsyncStar && analysis.hasYield) {
+      inits.add(makeInit(nextWhenCanceledName, null));
+    }
     if (analysis.hasExplicitReturns && isAsync) {
       inits.add(makeInit(returnValueName, null));
     }
     if (analysis.hasThis && !isSyncStar) {
       // Sync* functions must remember `this` on the level of the outer
       // function.
       inits.add(makeInit(selfName, new js.This()));
     }
     inits.addAll(localVariables.map((js.VariableDeclaration decl) {
       return new js.VariableInitialization(decl, null);
@@ skipping 1434 matching lines @@
     return condition || body;
   }
 
   @override
   bool visitDartYield(js.DartYield node) {
     hasYield = true;
     visit(node.expression);
     return true;
   }
 }