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

tests/compiler/dart2js/diagnose_ambiguous_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.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_source_file_helper.dart';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart'
        show Diagnostic;
 
 main() {
-  Uri script = currentDirectory.resolveUri(Platform.script);
+  Uri script = currentDirectory.resolve(nativeToUriPath(Platform.script));
   Uri libraryRoot = script.resolve('../../../sdk/');
   Uri packageRoot = script.resolve('./packages/');
 
   var provider = new MemorySourceFileProvider(MEMORY_SOURCE_FILES);
   var diagnostics = [];
   void diagnosticHandler(Uri uri, int begin, int end,
                          String message, Diagnostic kind) {
     if (kind == Diagnostic.VERBOSE_INFO) {
       return;
     }
@@ skipping 44 matching lines @@
 hest() {}
 """,
   'exporter.dart': """
 library exporter;
 
 export 'library.dart';
 
 hest() {}
 """,
 };