Remove Encoding-enum from dart:io and add interface in dart:convert.

tests/standalone/vmservice/test_helper.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 vmservice_test_helper;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'dart:json' as JSON;
@@ skipping 79 matching lines @@
     String dartExecutable = Platform.executable;
     print('** Launching $scriptPath');
     return Process.start(dartExecutable,
                          ['--enable-vm-service:0', scriptPath]).then((p) {
 
       Completer completer = new Completer();
       process = p;
       var portNumber;
       var blank;
       var first = true;
-      process.stdout.transform(new StringDecoder())
+      process.stdout.transform(UTF8.decoder)
                     .transform(new LineSplitter()).listen((line) {
         if (line.startsWith('VmService listening on port ')) {
           RegExp portExp = new RegExp(r"\d+");
           var port = portExp.stringMatch(line);
           portNumber = int.parse(port);
         }
         if (line == '') {
           // Received blank line.
           blank = true;
         }
         if (portNumber != null && blank == true && first == true) {
           completer.complete(portNumber);
           // Stop repeat completions.
           first = false;
           print('** Signaled to run test queries on $portNumber');
         }
         print(line);
       });
-      process.stderr.transform(new StringDecoder())
+      process.stderr.transform(UTF8.decoder)
                     .transform(new LineSplitter()).listen((line) {
         print(line);
       });
       process.exitCode.then((code) {
         Expect.equals(0, code, 'Launched dart executable exited with error.');
       });
       return completer.future;
     });
   }
 
@@ skipping 44 matching lines @@
       if (isolate['id'] == id) {
         exists = true;
         Expect.isTrue(isolate['name'].startsWith(name),
                       'Isolate $id does not have name prefix: $name'
                       ' (was ${isolate['name']})');
       }
     });
     Expect.isTrue(exists, 'No isolate with id: $id');
   }
 }