Revert "Add StringSink and update StringBuffer."

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 {
+patch /* abstract */ class StringBuffer {
+  /* patch */ factory StringBuffer([Object content = ""])
+    => new _StringBufferImpl(content);
+}
+
+class _StringBufferImpl implements StringBuffer {
+
   List<String> _buffer;
   int _length;
 
   /// Creates the string buffer with an initial content.
-  /* patch */ StringBuffer([Object content = ""]) {
-    // TODO(floitsch): remove or rewrite this call to clear. It is currently
-    // necessary because the VM doesn't allow default values for fields in
-    // patch classes.
-    _buffer = new List<String>();
-    _length = 0;
-    write(content);
+  _StringBufferImpl(Object content) {
+    clear();
+    add(content);
   }
 
-  /* patch */ int get length => _length;
+  /// Returns the length of the buffer.
+  int get length => _length;
+
+  bool get isEmpty => _length == 0;
 
   /// Adds [obj] to the buffer.
-  /* patch */ void _write(String str) {
+  void add(Object obj) {
+    // TODO(srdjan): The following four lines could be replaced by
+    // '$obj', but apparently this is too slow on the Dart VM.
+    String str = obj.toString();
+    if (str is !String) {
+      throw new ArgumentError('toString() did not return a string');
+    }
+    if (str.isEmpty) return;
     _buffer.add(str);
     _length += str.length;
   }
 
+  /// Adds all items in [objects] to the buffer.
+  void addAll(Iterable objects) {
+    for (Object obj in objects) add(obj);
+  }
+
+  /// Adds the string representation of [charCode] to the buffer.
+  void addCharCode(int charCode) {
+    add(new String.fromCharCodes([charCode]));
+  }
+
   /// Clears the string buffer.
-  /* patch */ void clear() {
+  void clear() {
     _buffer = new List<String>();
     _length = 0;
   }
 
   /// Returns the contents of buffer as a concatenated string.
-  /* patch */ String toString() {
+  String toString() {
     if (_buffer.length == 0) return "";
     if (_buffer.length == 1) return _buffer[0];
     String result = Strings.concatAll(_buffer);
     _buffer.clear();
     _buffer.add(result);
     // Since we track the length at each add operation, there is no
     // need to update it in this function.
     return result;
   }
 }