Various DDC fixes for windows

pkg/dev_compiler/test/testing.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 dev_compiler.src.testing;
 
 import 'dart:mirrors';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:path/path.dart' as path;
 
-final String testDirectory =
-    path.dirname((reflectClass(_TestUtils).owner as LibraryMirror).uri.path);
+final String testingPath =
+    path.fromUri((reflectClass(_TestUtils).owner as LibraryMirror).uri);
+final String testDirectory = path.dirname(testingPath);
 
 /// The local path to the root directory of the dev_compiler repo.
 final String repoDirectory = path.dirname(testDirectory);
 
 class _TestUtils {}
 
 class TestUriResolver extends ResourceUriResolver {
   final MemoryResourceProvider provider;
   TestUriResolver(provider)
       : provider = provider,
         super(provider);
 
   @override
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     if (uri.scheme == 'package') {
       return (provider.getResource('/packages/' + uri.path) as File)
           .createSource(uri);
     }
     return super.resolveAbsolute(uri, actualUri);
   }
 }