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

tests/standalone/io/process_run_output_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.
 //
 // Test script for testing that output is handled correctly for
 // non-interactive processes started with Process.run.
 
 import "package:expect/expect.dart";
+import "dart:convert";
 import "dart:io";
 import "process_test_util.dart";
 
-checkOutput(encoding, output) {
+checkOutput(String encoding, output) {
   if (encoding == 'ascii') {
     Expect.equals(output, 'abc');
   } else if (encoding == 'latin1') {
     Expect.equals(output, 'æøå');
   } else if (encoding == 'utf8') {
     Expect.listEquals(output.codeUnits, [955]);
   } else if (encoding == 'binary') {
     print(output);
     Expect.listEquals(output, [0, 1, 2]);
   }
 }
 
-test(scriptFile, encoding, stream) {
+test(scriptFile, String encoding, stream) {
   var enc;
   if (encoding == 'ascii') {
-    enc = Encoding.ASCII;
+    enc = ASCII;
   } else if (encoding == 'latin1') {
-    enc = Encoding.ISO_8859_1;
+    enc = LATIN1;
   } else if (encoding == 'utf8') {
-    enc = Encoding.UTF_8;
+    enc = UTF8;
   } else if (encoding == 'binary') {
     enc = null;
   }
 
   if (stream == 'stdout') {
     Process.run(Platform.executable,
                 [scriptFile, encoding, stream],
                 stdoutEncoding: enc). then((result) {
       Expect.equals(result.exitCode, 0);
       Expect.equals(result.stderr, '');
@@ skipping 19 matching lines @@
   Expect.isTrue(scriptFile.existsSync());
   test(scriptFile.path, 'ascii', 'stdout');
   test(scriptFile.path, 'ascii', 'stderr');
   test(scriptFile.path, 'latin1', 'stdout');
   test(scriptFile.path, 'latin1', 'stderr');
   test(scriptFile.path, 'utf8', 'stdout');
   test(scriptFile.path, 'utf8', 'stderr');
   test(scriptFile.path, 'binary', 'stdout');
   test(scriptFile.path, 'binary', 'stderr');
 }