Shoehorn dart2js into the SDK.

dart/lib/compiler/implementation/dart2js.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('dart2js');
 
 #import('dart:io');
 #import('dart:uri');
 #import('dart:utf');
 
@@ skipping 120 matching lines @@
   file.closeSync();
   return new String.fromCharCodes(new Utf8Decoder(buffer).decodeRest());
 }
 
 void fail(String message) {
   print(message);
   exit(1);
 }
 
 void compilerMain(Options options) {
-  List<String> argv = ['--library-root=${options.script}/$LIBRARY_ROOT'];
+  var root = uriPathToNative("/$LIBRARY_ROOT");
+  List<String> argv = ['--library-root=${options.script}$root'];
   argv.addAll(options.arguments);
   compile(argv);
 }
 
 void main() {
   try {
     compilerMain(new Options());
   } catch (var exception, var trace) {
     try {
       print('Internal error: $exception');
     } catch (var ignored) {
       print('Internal error: error while printing exception');
     }
     try {
       print(trace);
     } finally {
       exit(253); // 253 is recognized as a crash by our test scripts.
     }
   }
 }