Fix some TODOs

runtime/lib/string_buffer_patch.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.
 
 patch class StringBuffer {
   static const int _BUFFER_SIZE = 64;
   static const int _PARTS_TO_COMPACT = 128;
   static const int _PARTS_TO_COMPACT_SIZE_LIMIT = _PARTS_TO_COMPACT * 8;
 
   /**
@@ skipping 37 matching lines @@
   int _bufferCodeUnitMagnitude = 0;
 
   /// Creates the string buffer with an initial content.
   /* patch */ StringBuffer([Object content = ""]) {
     write(content);
   }
 
   /* patch */ int get length => _partsCodeUnits + _bufferPosition;
 
   /* patch */ void write(Object obj) {
-    String str;
-    if (obj is String) {
-      str = obj;
-    } else {
-      // TODO(srdjan): The following four lines could be replaced by
-      // '$obj', but apparently this is too slow on the Dart VM.
-      str = obj.toString();
-      if (str is! String) {
-        throw new ArgumentError('toString() did not return a string');
-      }
-    }
+    String str = '$obj';
     if (str.isEmpty) return;
     _consumeBuffer();
     _addPart(str);
   }
 
   /* patch */ void writeCharCode(int charCode) {
     if (charCode <= 0xFFFF) {
       if (charCode < 0) {
         throw new RangeError.range(charCode, 0, 0x10FFFF);
       }
@@ skipping 106 matching lines @@
     _partsCodeUnitsSinceCompaction = 0;
     _partsCompactionIndex = _parts.length;
   }
 
   /**
    * Create a [String] from the UFT-16 code units in buffer.
    */
   static String _create(Uint16List buffer, int length, bool isLatin1)
       native "StringBuffer_createStringFromUint16Array";
 }