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

tests/standalone/io/file_error_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.
 //
 // Dart test program for testing error handling in file I/O.
 
 import "package:expect/expect.dart";
+import "dart:convert";
 import "dart:io";
 import "dart:isolate";
 
 Directory tempDir() {
   return new Directory('').createTempSync();
 }
 
 
 bool checkNonExistentFileException(e, str) {
   Expect.isTrue(e is FileException);
@@ skipping 180 matching lines @@
   p.receive((x, y) {
     p.close();
     temp.deleteSync(recursive: true);
   });
   var file = new File("${temp.path}/nonExistentFile4");
 
   // Non-existing file should throw exception.
   Expect.throws(() => file.readAsStringSync(),
                 (e) => checkOpenNonExistentFileException(e));
 
-  var readAsStringFuture = file.readAsString(encoding: Encoding.ASCII);
+  var readAsStringFuture = file.readAsString(encoding: ASCII);
   readAsStringFuture.then((data) => Expect.fail("Unreachable code"))
   .catchError((error) {
     checkOpenNonExistentFileException(error);
     p.toSendPort().send(null);
   });
 }
 
 testReadAsLinesNonExistent() {
   Directory temp = tempDir();
   ReceivePort p = new ReceivePort();
   p.receive((x, y) {
     p.close();
     temp.deleteSync(recursive: true);
   });
   var file = new File("${temp.path}/nonExistentFile5");
 
   // Non-existing file should throw exception.
   Expect.throws(() => file.readAsLinesSync(),
                 (e) => checkOpenNonExistentFileException(e));
 
-  var readAsLinesFuture = file.readAsLines(encoding: Encoding.ASCII);
+  var readAsLinesFuture = file.readAsLines(encoding: ASCII);
   readAsLinesFuture.then((data) => Expect.fail("Unreachable code"))
   .catchError((error) {
     checkOpenNonExistentFileException(error);
     p.toSendPort().send(null);
   });
 }
 
 bool checkWriteReadOnlyFileException(e) {
   Expect.isTrue(e is FileException);
   Expect.isTrue(e.osError != null);
@@ skipping 214 matching lines @@
   testReadAsLinesNonExistent();
   testWriteByteToReadOnlyFile();
   testWriteFromToReadOnlyFile();
   testTruncateReadOnlyFile();
   testOperateOnClosedFile();
   testRepeatedlyCloseFile();
   testRepeatedlyCloseFileSync();
   testReadSyncBigInt();
   testReadSyncClosedFile();
 }