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

tools/coverage.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 a dart script as
 // a debug target, single stepping through the entire program, and
 // recording each breakpoint. At the end, a coverage map of the source
 // is printed.
 //
 // Usage: dart coverage.dart [--wire] [--verbose] target_script.dart
@@ skipping 196 matching lines @@
   bool shutdownEventSeen = false;
   int isolateId = 0;
   int libraryId = null;
 
   int nextMessageId = 0;
   bool isPaused = false;
   bool pendingAck = false;
 
   Debugger(this.targetProcess) {
     var stdoutStringStream = targetProcess.stdout
-        .transform(new StringDecoder())
+        .transform(UTF8.decoder)
         .transform(new LineSplitter());
     stdoutStringStream.listen((line) {
       if (showDebuggeeOutput) {
         print("TARG: $line");
       }
       if (line.startsWith("Debugger listening")) {
         RegExp portExpr = new RegExp(r"\d+");
         var port = portExpr.stringMatch(line);
         var pid = targetProcess.pid;
         print("Coverage target process (pid $pid) found "
               "listening on port $port.");
         openConnection(int.parse(port));
       }
     });
 
     var stderrStringStream = targetProcess.stderr
-        .transform(new StringDecoder())
+        .transform(UTF8.decoder)
         .transform(new LineSplitter());
     stderrStringStream.listen((line) {
       if (showDebuggeeOutput) {
         print("TARG: $line");
       }
     });
   }
 
   // Handle debugger events, updating the debugger state.
   void handleEvent(Map<String,dynamic> msg) {
@@ skipping 120 matching lines @@
 
   // Record error message.
   void error(String s) {
     errors.add(s);
   }
 
   void openConnection(int portNumber) {
     Socket.connect("127.0.0.1", portNumber).then((s) {
       socket = s;
       socket.setOption(SocketOption.TCP_NODELAY, true);
-      var stringStream = socket.transform(new StringDecoder());
+      var stringStream = socket.transform(UTF8.decoder);
       stringStream.listen(
           (str) {
             try {
               responses.append(str);
               handleMessages();
             } catch(e, trace) {
               print("Unexpected exception:\n$e\n$trace");
               cleanup();
             }
           },
@@ skipping 154 matching lines @@
         targetOpts.add(str);
         break;
     }
   }
 
   Process.start(options.executable, targetOpts).then((Process process) {
     process.stdin.close();
     debugger = new Debugger(process);
   });
 }