Properly convert paths to package root URIs.

lib/src/util/io.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.io;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:mirrors';
 
@@ skipping 82 matching lines @@
 /// [LoadException] is thrown.
 String packageRootFor(String path, [String override]) {
   var packageRoot = override == null
       ? p.join(p.dirname(path), 'packages')
       : override;
 
   if (!new Directory(packageRoot).existsSync()) {
     throw new LoadException(path, "Directory $packageRoot does not exist.");
   }
 
-  return packageRoot;
+  return p.toUri(packageRoot).toString();
 }
 
 /// The library name must be globally unique, or the wrong library path may be
 /// returned.
 String libraryPath(Symbol libraryName, {String packageRoot}) {
   var lib = currentMirrorSystem().findLibrary(libraryName);
   if (lib.uri.scheme != 'package') return p.fromUri(lib.uri);
 
   // TODO(nweiz): is there a way to avoid assuming this is being run next to a
   // packages directory?.
   if (packageRoot == null) packageRoot = p.absolute('packages');
   return p.join(packageRoot, p.fromUri(lib.uri.path));
 }