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

tests/standalone/coverage_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.
 
 // This test forks a second vm process that runs the script tools/coverage.dart
 // and verifies that the coverage tool produces its expected output.
 // This test is mainly here to ensure that the coverage tool compiles and
 // runs.
 
 import "dart:async";
@@ skipping 47 matching lines @@
 
   sourceLines = new File(targPath).readAsLinesSync();
   assert(sourceLines != null);
 
   var processOpts = [ "--compile_all", toolPath, targPath ];
 
   Process.start(Platform.executable, processOpts).then((Process process) {
     coverageToolProcess = process;
     coverageToolProcess.stdin.close();
     var stdoutStringStream = coverageToolProcess.stdout
-        .transform(new StringDecoder())
+        .transform(UTF8.decoder)
         .transform(new LineSplitter());
 
     var stderrStringStream = coverageToolProcess.stderr
-        .transform(new StringDecoder())
+        .transform(UTF8.decoder)
         .transform(new LineSplitter());
 
     // Wait for 3 future events: stdout and stderr streams of the coverage
     // tool process closed, and coverage tool process terminated.
     var futures = [];
     var subscription = stdoutStringStream.listen(onCoverageOutput);
     futures.add(subscription.asFuture(true));
     subscription = stderrStringStream.listen(onCoverageOutput);
     futures.add(subscription.asFuture(true));
     futures.add(coverageToolProcess.exitCode.then(checkExitCode));
     Future.wait(futures).then((results) {
       checkSuccess();
       if (results.contains(false)) exit(-1);
     });
   });
 }