Rename File.readAsText to File.readAsString. There is no Text type in Dart

tests/standalone/io/file_non_ascii_sync_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.
 
 import 'dart:io';
 
 main() {
   Directory scriptDir = new File(new Options().script).directorySync();
   var f = new File("${scriptDir.path}/æøå/æøå.dat");
   // On MacOS you get the decomposed utf8 form of file and directory
   // names from the system. Therefore, we have to check for both here.
   var precomposed = 'æøå';
   var decomposed = new String.fromCharCodes([47, 230, 248, 97, 778]);
   Expect.isTrue(f.existsSync());
   f.createSync();
   var path = f.directorySync().path;
   Expect.isTrue(f.directorySync().path.endsWith(precomposed) ||
                 f.directorySync().path.endsWith(decomposed));
   Expect.equals(6, f.lengthSync());
   f.lastModifiedSync();
   Expect.isTrue(f.fullPathSync().endsWith('${precomposed}.dat') ||
                 f.fullPathSync().endsWith('${decomposed}.dat'));
   // The contents of the file is precomposed utf8.
-  Expect.equals(precomposed, f.readAsTextSync());
+  Expect.equals(precomposed, f.readAsStringSync());
 }