Don't show dart2js package warnings.

lib/src/runner/browser/compiler_pool.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 test.util.compiler_pool;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 53 matching lines @@
 import "${p.toUri(p.absolute(dartPath))}" as test;
 
 void main(_) {
   IframeListener.start(() => test.main);
 }
 ''');
 
         var dart2jsPath = p.join(sdkDir, 'bin', 'dart2js');
         if (Platform.isWindows) dart2jsPath += '.bat';
 
-        var args = ["--checked", wrapperPath, "--out=$jsPath", "--show-package-warnings"];
+        var args = ["--checked", wrapperPath, "--out=$jsPath"];
 
         if (packageRoot != null) {
           args.add("--package-root=${p.toUri(p.absolute(packageRoot))}");
         }
 
         if (_color) args.add("--enable-diagnostic-colors");
 
         var process = await Process.start(dart2jsPath, args);
         if (_closed) {
           process.kill();
@@ skipping 58 matching lines @@
   /// have been killed and all resources released.
   Future close() {
     return _closeMemo.runOnce(() async {
       await Future.wait(_processes.map((process) async {
         process.kill();
         await process.exitCode;
       }));
     });
   }
 }