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

tests/standalone/io/test_extension_test.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.
 //
 // Dart test program for testing native extensions.
 
 import "package:expect/expect.dart";
 import "package:path/path.dart";
 import 'dart:async';
 import 'dart:io';
@@ skipping 18 matching lines @@
     case 'macos':
       return join(buildDirectory, 'libtest_extension.dylib');
     case 'windows':
       return join(buildDirectory, 'test_extension.dll');
     default:
       Expect.fail('Unknown operating system ${Platform.operatingSystem}');
   }
 }
 
 void main() {
-  String scriptDirectory = dirname(Platform.script.toFilePath());
+  String scriptDirectory = dirname(Platform.script);
   String buildDirectory = dirname(Platform.executable);
   Directory tempDirectory =
       Directory.systemTemp.createTempSync('dart_test_extension');
   String testDirectory = tempDirectory.path;
 
   // Copy test_extension shared library, test_extension.dart and
   // test_extension_tester.dart to the temporary test directory.
   copyFileToDirectory(getExtensionPath(buildDirectory),
                       testDirectory).then((_) {
     var extensionDartFile = join(scriptDirectory, 'test_extension.dart');
     return copyFileToDirectory(extensionDartFile, testDirectory);
   }).then((_) {
     var testExtensionTesterFile =
         join(scriptDirectory, 'test_extension_tester.dart');
     return copyFileToDirectory(testExtensionTesterFile, testDirectory);
   }).then((_) {
     var script = join(testDirectory, 'test_extension_tester.dart');
     return Process.run(Platform.executable, [script]);
   })..then((ProcessResult result) {
     Expect.equals(0, result.exitCode);
     tempDirectory.deleteSync(recursive: true);
   })..catchError((_) {
     tempDirectory.deleteSync(recursive: true);
   });
 }