Remove dart:codec and move classes into dart:convert.

tests/lib/convert/utf82_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.
 
 library utf8_test;
 import "package:expect/expect.dart";
 import 'dart:convert';
-import 'dart:codec';
 
 String decode(List<int> bytes) => new Utf8Decoder().convert(bytes);
 String decodeAllowMalformed(List<int> bytes) {
   return new Utf8Decoder(allowMalformed: true).convert(bytes);
 }
 
 String decode2(List<int> bytes) => UTF8.decode(bytes);
 String decodeAllowMalformed2(List<int> bytes) {
   return UTF8.decode(bytes, allowMalformed: true);
 }
@@ skipping 107 matching lines @@
     Expect.throws(() => decode3(bytes), (e) => e is FormatException);
     Expect.throws(() => decode4(bytes), (e) => e is FormatException);
 
     String expected = test[1];
     Expect.equals(expected, decodeAllowMalformed(bytes));
     Expect.equals(expected, decodeAllowMalformed2(bytes));
     Expect.equals(expected, decodeAllowMalformed3(bytes));
     Expect.equals(expected, decodeAllowMalformed4(bytes));
   }
 }