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

tests/standalone/io/file_non_ascii_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';
 import 'dart:isolate';
 
 main() {
   var port = new ReceivePort();
   Directory scriptDir = new File(new Options().script).directorySync();
   var f = new File("${scriptDir.path}/æøå/æøå.dat");
   // On MacOS you get the decomposed utf8 form of file and directory
   // names from the system. Therefore, we have to check for both here.
   var precomposed = 'æøå';
   var decomposed = new String.fromCharCodes([47, 230, 248, 97, 778]);
   f.exists().then((e) {
     Expect.isTrue(e);
     f.create().then((_) {
       f.directory().then((d) {
         Expect.isTrue(d.path.endsWith(precomposed) ||
                       d.path.endsWith(decomposed));
         f.length().then((l) {
           Expect.equals(6, l);
           f.lastModified().then((_) {
             f.fullPath().then((p) {
               Expect.isTrue(p.endsWith('${precomposed}.dat') ||
                             p.endsWith('${decomposed}.dat'));
-              f.readAsText().then((contents) {
+              f.readAsString().then((contents) {
                 Expect.equals(precomposed, contents);
                 port.close();
               });
             });
           });
         });
       });
     });
   });
 }