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

pkg/barback/test/asset_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.
 
 library barback.test.asset_test;
 
 import 'dart:async';
+import 'dart:convert' show Encoding, UTF8, LATIN1;
 import 'dart:io';
 import 'dart:utf';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as pathos;
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 /// The contents of the test binary file.
@@ skipping 83 matching lines @@
   group("readAsString()", () {
     group("byte asset", () {
       test("defaults to UTF-8 if encoding is omitted", () {
         var asset = new Asset.fromBytes(id, encodeUtf8("çøñ†éℵ™"));
         expect(asset.readAsString(),
             completion(equals("çøñ†éℵ™")));
       });
 
       test("supports UTF-8", () {
         var asset = new Asset.fromBytes(id, encodeUtf8("çøñ†éℵ™"));
-        expect(asset.readAsString(encoding: Encoding.UTF_8),
+        expect(asset.readAsString(encoding: UTF8),
             completion(equals("çøñ†éℵ™")));
       });
 
       // TODO(rnystrom): Test other encodings once #6284 is fixed.
     });
 
     group("string asset", () {
       test("gets the string", () {
         var asset = new Asset.fromString(id, "contents");
         expect(asset.readAsString(),
             completion(equals("contents")));
       });
 
       test("ignores the encoding", () {
         var asset = new Asset.fromString(id, "contents");
-        expect(asset.readAsString(encoding: Encoding.ISO_8859_1),
+        expect(asset.readAsString(encoding: LATIN1),
             completion(equals("contents")));
       });
     });
 
     group("file asset", () {
       test("defaults to UTF-8 if encoding is omitted", () {
         var asset = new Asset.fromPath(id, textFilePath);
         expect(asset.readAsString(),
             completion(equals("çøñ†éℵ™")));
       });
@@ skipping 33 matching lines @@
     });
 
     group("file asset", () {
       test("shows the file path", () {
         var asset = new Asset.fromPath(id, "path.txt");
         expect(asset.toString(), equals('File "path.txt"'));
       });
     });
   });
 }