*Properly* fix that package-root bug.

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:collection';
 import 'dart:io';
 
@@ skipping 55 matching lines @@
   IframeListener.start(() => test.main);
 }
 ''');
 
         var dart2jsPath = p.join(sdkDir, 'bin', 'dart2js');
         if (Platform.isWindows) dart2jsPath += '.bat';
 
         var args = ["--checked", wrapperPath, "--out=$jsPath"];
 
         if (packageRoot != null) {
-          args.add("--package-root=${p.absolute(packageRoot)}");
+          args.add("--package-root=${p.toUri(p.absolute(packageRoot))}");
         }
 
         if (_color) args.add("--enable-diagnostic-colors");
 
         return Process.start(dart2jsPath, args).then((process) {
           var compiler = new _Compiler(dartPath, process);
 
           if (_compilers.isEmpty) _showProcess(compiler);
           _compilers.add(compiler);
 
@@ skipping 42 matching lines @@
   /// The underlying process.
   final Process process;
 
   /// A future that will complete once this instance has finished running and
   /// all its output has been printed.
   Future get onDone => onDoneCompleter.future;
   final onDoneCompleter = new Completer();
 
   _Compiler(this.path, this.process);
 }