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

tests/lib/convert/encoding_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.
+
+import "package:expect/expect.dart";
+import 'dart:async';
+import 'dart:convert';
+import 'unicode_tests.dart';
+import '../../async_helper.dart';
+
+void runTest(List<int> bytes, expected) {
+  var controller = new StreamController();
+  asyncStart();
+  UTF8.decodeStream(controller.stream).then((decoded) {
+    Expect.equals(expected, decoded);
+    asyncEnd();
+  });
+  int i = 0;
+  while (i < bytes.length) {
+    List nextChunk = [];
+    for (int j = 0; j < 3; j++) {
+      if (i < bytes.length) {
+        nextChunk.add(bytes[i]);
+        i++;
+      }
+    }
+    controller.add(nextChunk);
+  }
+  controller.close();
+}
+
+
+main() {
+  for (var test in UNICODE_TESTS) {
+    var bytes = test[0];
+    var expected = test[1];
+    runTest(bytes, expected);
+  }
+}