Add StringSink and update StringBuffer.

sdk/lib/core/string_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.core;
+
+abstract class StringSink {
+
+  /**
+   * Converts [obj] to a String by invoking [:toString:] and adds the result to
+   * this [StringSink].
+   */
+  void write(Object obj);
+
+  /**
+   * Iterates over the given [objects] and [write]s them in sequence.
+   */
+  void writeAll(Iterable objects);
+
+  /**
+   * Converts [obj] to a String by invoking [:toString:] and adds the result to
+   * this [StringSink]. Then adds a new line.
+   */
+  void writeln(Object obj);
+
+  /**
+   * Writes the [charCode] to this [StringSink].
+   *
+   * This method is equivalent to [:write(new String.fromCharCode(charCode)):].
+   */
+  void writeCharCode(int charCode);
+}