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

tests/standalone/io/file_input_stream_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.
 // Testing file input stream, VM-only, standalone test.
 
 import "dart:convert";
 import "dart:io";
 import "dart:isolate";
 
 import "package:expect/expect.dart";
 
 // Helper method to be able to run the test from the runtime
 // directory, or the top directory.
 String getFilename(String path) =>
     new File(path).existsSync() ? path : '../$path';
 
 void testStringLineSplitter() {
   String fileName = getFilename("tests/standalone/io/readuntil_test.dat");
   // File contains "Hello Dart\nwassup!\n"
   File file = new File(fileName);
   int linesRead = 0;
   var lineStream = file.openRead()
-    .transform(new StringDecoder())
+    .transform(UTF8.decoder)
     .transform(new LineSplitter());
   lineStream.listen((line) {
     linesRead++;
     if (linesRead == 1) {
       Expect.equals("Hello Dart", line);
     } else if (linesRead == 2) {
       Expect.equals("wassup!", line);
     } else {
       Expect.fail("More or less than 2 lines read ($linesRead lines read).");
     }
@@ skipping 191 matching lines @@
   test(null, -1);
 }
 
 
 void testStringLineSplitterEnding(String name, int length) {
   String fileName = getFilename("tests/standalone/io/$name");
   // File contains 10 lines.
   File file = new File(fileName);
   Expect.equals(length, file.openSync().lengthSync());
   var lineStream = file.openRead()
-    .transform(new StringDecoder())
+    .transform(UTF8.decoder)
     .transform(new LineSplitter());
   int lineCount = 0;
   lineStream.listen(
       (line) {
         lineCount++;
         Expect.isTrue(lineCount <= 10);
         if (line[0] != "#") {
           Expect.equals("Line $lineCount", line);
         }
       },
@@ skipping 10 matching lines @@
   testInputStreamDelete();
   testInputStreamAppend();
   testInputStreamOffset();
   testInputStreamBadOffset();
   // Check the length of these files as both are text files where one
   // is without a terminating line separator which can easily be added
   // back if accidentally opened in a text editor.
   testStringLineSplitterEnding("readline_test1.dat", 111);
   testStringLineSplitterEnding("readline_test2.dat", 114);
 }