Use window.location.href in path under dart2js.

pkg/path/lib/path.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.
 
 /// A comprehensive, cross-platform path manipulation library.
 ///
 /// ## Installing ##
 ///
 /// Use [pub][] to install this package. Add the following to your
 /// `pubspec.yaml` file.
 ///
 ///     dependencies:
 ///       path: any
 ///
 /// Then run `pub install`.
 ///
 /// For more information, see the [path package on pub.dartlang.org][pkg].
 ///
 /// [pub]: http://pub.dartlang.org
 /// [pkg]: http://pub.dartlang.org/packages/path
 library path;
 
+@MirrorsUsed(targets: 'dart.dom.html.window, '
+    'dart.io.Directory.current, '
+    'dart.io.Platform.operatingSystem')
 import 'dart:mirrors';
 
 /// An internal builder for the current OS so we can provide a straight
 /// functional interface and not require users to create one.
 final _builder = new Builder();
 
 /**
  * Inserts [length] elements in front of the [list] and fills them with the
  * [fillValue].
  */
 void _growListFront(List list, int length, fillValue) =>
   list.insertAll(0, new List.filled(length, fillValue));
 
 /// If we're running in the server-side Dart VM, this will return a
 /// [LibraryMirror] that gives access to the `dart:io` library.
 ///
 /// If `dart:io` is not available, this returns null.
-LibraryMirror get _io {
-  try {
-    return currentMirrorSystem().libraries[Uri.parse('dart:io')];
-  } catch (_) {
-    return null;
-  }
-}
+LibraryMirror get _io => currentMirrorSystem().libraries[Uri.parse('dart:io')];
 
 // TODO(nweiz): when issue 6490 or 6943 are fixed, make this work under dart2js.
 /// If we're running in Dartium, this will return a [LibraryMirror] that gives
 /// access to the `dart:html` library.
 ///
 /// If `dart:html` is not available, this returns null.
-LibraryMirror get _html {
-  try {
-    return currentMirrorSystem().libraries[Uri.parse('dart:html')];
-  } catch (_) {
-    return null;
-  }
-}
+LibraryMirror get _html =>
+  currentMirrorSystem().libraries[Uri.parse('dart:html')];
 
 /// Gets the path to the current working directory.
 ///
 /// In the browser, this means the current URL. When using dart2js, this
 /// currently returns `.` due to technical constraints. In the future, it will
 /// return the current URL.
 String get current {
   if (_io != null) {
     return _io.classes[const Symbol('Directory')]
         .getField(const Symbol('current')).reflectee.path;
@@ skipping 1061 matching lines @@
     // doesn't count.
     if (lastDot <= 0) return [file, ''];
 
     return [file.substring(0, lastDot), file.substring(lastDot)];
   }
 
   _ParsedPath clone() => new _ParsedPath(
       style, root, isRootRelative,
       new List.from(parts), new List.from(separators));
 }