Fix some TODOs

sdk/lib/io/io_sink.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.
 
 part of dart.io;
 
 /**
  * Helper class to wrap a [StreamConsumer<List<int>>] and provide
  * utility functions for writing to the StreamConsumer directly. The
  * [IOSink] buffers the input given by all [StringSink] methods and will delay
@@ skipping 255 matching lines @@
   Encoding get encoding => _encoding;
 
   void set encoding(Encoding value) {
     if (!_encodingMutable) {
       throw new StateError("IOSink encoding is not mutable");
     }
     _encoding = value;
   }
 
   void write(Object obj) {
-    // This comment is copied from runtime/lib/string_buffer_patch.dart.
-    // TODO(srdjan): The following four lines could be replaced by
-    // '$obj', but apparently this is too slow on the Dart VM.
-    String string;
-    if (obj is String) {
-      string = obj;
-    } else {
-      string = obj.toString();
-      if (string is! String) {
-        throw new ArgumentError('toString() did not return a string');
-      }
-    }
+    String string = '$obj';
     if (string.isEmpty) return;
     add(_encoding.encode(string));
   }
 
   void writeAll(Iterable objects, [String separator = ""]) {
     Iterator iterator = objects.iterator;
     if (!iterator.moveNext()) return;
     if (separator.isEmpty) {
       do {
         write(iterator.current);
       } while (iterator.moveNext());
     } else {
       write(iterator.current);
       while (iterator.moveNext()) {
         write(separator);
         write(iterator.current);
       }
     }
   }
 
   void writeln([Object obj = ""]) {
     write(obj);
     write("\n");
   }
 
   void writeCharCode(int charCode) {
     write(new String.fromCharCode(charCode));
   }
 }