dart2js: add --preserve-uris flag.

tests/lib/mirrors/libraries_test.dart

 // Copyright (c) 2013, 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.libraries_test;
 
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 main() {
   MirrorSystem mirrors = currentMirrorSystem();
   Expect.isNotNull(mirrors, 'mirrors is null');
 
   Map<Uri, LibraryMirror> libraries = mirrors.libraries;
   Expect.isNotNull(libraries, 'libraries is null');
 
+  Expect.isTrue(libraries.isNotEmpty);
   LibraryMirror mirrorsLibrary = libraries[Uri.parse('dart:mirrors')];
-  Expect.isNotNull(mirrorsLibrary, 'mirrorsLibrary is null');
+  if (mirrorsLibrary == null) {
+    // In minified mode we don't preserve the URIs.
+    mirrorsLibrary = libraries.values
+        .firstWhere((LibraryMirror lm) => lm.simpleName == #dart.mirrors);
+    Uri uri = mirrorsLibrary.uri;
+    Expect.equals("https", uri.scheme);
+    Expect.equals("dartlang.org", uri.host);
+    Expect.equals("/dart2js-stripped-uri", uri.path);
+  }
 
   ClassMirror cls = mirrorsLibrary.declarations[#LibraryMirror];
   Expect.isNotNull(cls, 'cls is null');
 
   Expect.equals(#dart.mirrors.LibraryMirror, cls.qualifiedName);
   Expect.equals(reflectClass(LibraryMirror), cls);
 }