Change dart:io Platform.script to return a Uri.

sdk/lib/_internal/compiler/implementation/dart2js.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 dart2js.cmdline;
 
 import 'dart:async';
 import 'dart:io'
     show exit, File, FileMode, Platform, RandomAccessFile;
 import 'dart:math' as math;
@@ skipping 436 matching lines @@
     diagnosticHandler.diagnosticHandler(
         null, -1, -1, message, api.Diagnostic.ERROR);
   } else {
     print(message);
   }
   exit(1);
 }
 
 Future compilerMain(List<String> arguments) {
   var root = uriPathToNative("/$LIBRARY_ROOT");
-  arguments =
-      <String>['--library-root=${Platform.script}$root']..addAll(arguments);
+  arguments = <String>['--library-root=${Platform.script.toFilePath()}$root']
+      ..addAll(arguments);
   return compile(arguments);
 }
 
 void help() {
   // This message should be no longer than 20 lines. The default
   // terminal size normally 80x24. Two lines are used for the prompts
   // before and after running the compiler. Another two lines may be
   // used to print an error message.
   print('''
 Usage: dart2js [options] dartfile
@@ skipping 130 matching lines @@
 
     try {
       if (trace != null) {
         print(trace);
       }
     } finally {
       exit(253); // 253 is recognized as a crash by our test scripts.
     }
   });
 }