Eliminate uses of fuzzy arrows in pkg/testing (fix bots).

pkg/testing/lib/src/zone_helper.dart

 // Copyright (c) 2016, 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.md file.
 
 /// Helper functions for running code in a Zone.
 library testing.zone_helper;
 
 import 'dart:async' show Completer, Future, ZoneSpecification, runZoned;
 
 import 'dart:io' show exit, stderr;
@@ skipping 30 matching lines @@
       stderr
           .write("$errorString\n" + (stackTrace == null ? "" : "$stackTrace"));
       stderr.flush();
       exit(255);
     }
   }
 
   ZoneSpecification specification = new ZoneSpecification(print: printWrapper);
 
   ReceivePort errorPort = new ReceivePort();
-  Future errorFuture = errorPort.listen((List errors) {
+  Future errorFuture = errorPort.listen((_errors) {
+    List errors = _errors;
     Isolate.current.removeErrorListener(errorPort.sendPort);
     errorPort.close();
     var error = errors[0];
     var stackTrace = errors[1];
     if (stackTrace != null) {
       stackTrace = new StackTrace.fromString(stackTrace);
     }
     handleUncaughtError(error, stackTrace);
   }).asFuture();
 
@@ skipping 17 matching lines @@
 /// [Isolate.addOnExitListener].
 Future acknowledgeControlMessages(Isolate isolate, {Capability resume}) {
   ReceivePort ping = new ReceivePort();
   Isolate.current.ping(ping.sendPort);
   if (resume == null) {
     return ping.first;
   } else {
     return ping.first.then((_) => isolate.resume(resume));
   }
 }