dart2js: add --preserve-uris flag.

tests/lib/mirrors/library_uri_package_test.dart

 // Copyright (c) 2011, 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.
 
 // Test library uri for a library read as a package .
 
 library MirrorsTest;
 
 import 'dart:mirrors';
 import 'package:args/args.dart';
 import 'package:unittest/unittest.dart';
 
 testLibraryUri(var value, Uri expectedUri) {
   var valueMirror = reflect(value);
   ClassMirror valueClass = valueMirror.type;
   LibraryMirror valueLibrary = valueClass.owner;
-  expect(valueLibrary.uri, equals(expectedUri));
+  Uri uri = valueLibrary.uri;
+  if (uri.scheme != "https" ||
+      uri.host != "dartlang.org" ||
+      uri.path != "/dart2js-stripped-uri") {
+    expect(uri, equals(expectedUri));
+  }
 }
 
 main() {
   var mirrors = currentMirrorSystem();
   test("Test package library uri", () {
     testLibraryUri(new ArgParser(),
         Uri.parse('package:args/src/arg_parser.dart'));
   });
 }