Move core and coreimpl from corelib/ to lib/core and lib/coreimpl.

corelib/src/string_buffer.dart

-// Copyright (c) 2011, 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.
-
-/**
- * The StringBuffer class is useful for concatenating strings
- * efficiently. Only on a call to [toString] are the strings
- * concatenated to a single String.
- */
-interface StringBuffer default StringBufferImpl {
-  /**
-   * Creates the string buffer with an initial content.
-   */
-  StringBuffer([Object content]);
-
-  /**
-   * Returns the length of the buffer.
-   */
-  int get length();
-
-  /**
-   * Returns whether the buffer is empty.
-   */
-  bool isEmpty();
-
-  /**
-   * Converts [obj] to a string and adds it to the buffer. Returns [:this:].
-   */
-  StringBuffer add(Object obj);
-
-  /**
-   * Adds the string representation of [charCode] to the buffer.
-   * Returns [this].
-   */
-  StringBuffer addCharCode(int charCode);
-
-  /**
-   * Adds all items in [objects] to the buffer. Returns [:this:].
-   */
-  StringBuffer addAll(Collection objects);
-
-  /**
-   * Clears the string buffer. Returns [:this:].
-   */
-  StringBuffer clear();
-
-  /**
-   * Returns the contents of buffer as a concatenated string.
-   */
-  String toString();
-}