Make dart2js work on Windows.

dart/lib/compiler/implementation/util/uri_extras.dart

+// Copyright (c) 2012, 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('uri_extras');
+
+#import('dart:uri');
+
+String relativize(Uri base, Uri uri) {
+  if (base.scheme == 'file' &&
+      base.scheme == uri.scheme &&
+      base.userInfo == uri.userInfo &&
+      base.domain == uri.domain &&
+      base.port == uri.port &&
+      uri.query == "" && uri.fragment == "") {
+    if (uri.path.startsWith(base.path)) {
+      return uri.path.substring(base.path.length);
+    }
+    List<String> uriParts = uri.path.split('/');
+    List<String> baseParts = base.path.split('/');
+    int common = 0;
+    int length = Math.min(uriParts.length, baseParts.length);
+    while (common < length && uriParts[common] == baseParts[common]) {
+      common++;
+    }
+    StringBuffer sb = new StringBuffer();
+    for (int i = common + 1; i < baseParts.length; i++) {
+      sb.add('../');
+    }
+    for (int i = common; i < uriParts.length - 1; i++) {
+      sb.add('${uriParts[i]}/');
+    }
+    sb.add('${uriParts.last()}');
+    return sb.toString();
+  }
+  return uri.toString();
+}