Move StringBufferImpl from coreimpl to core as _StringBufferImpl.

lib/core/string_buffer.dart

Index: lib/core/string_buffer.dart
diff --git a/lib/core/string_buffer.dart b/lib/core/string_buffer.dart
index 8095f6a0c367a3b05ade9b494bd9ca36ec1fba57..d56b3a3c7ffd2622595d95046da4979fa03cc144 100644
--- a/lib/core/string_buffer.dart
+++ b/lib/core/string_buffer.dart
@@ -11,7 +11,8 @@ abstract class StringBuffer {
   /**
    * Creates the string buffer with an initial content.
    */
-  factory StringBuffer([Object content = ""]) => new StringBufferImpl(content);
+  factory StringBuffer([Object content = ""])
+    => new _StringBufferImpl(content);
 
   /**
    * Returns the length of the buffer.
@@ -49,3 +50,81 @@ abstract class StringBuffer {
    */
   String toString();
 }
+
+class _StringBufferImpl implements StringBuffer {
+  /**
+   * Creates the string buffer with an initial content.
+   */
+  _StringBufferImpl(Object content) {
+    clear();
+    add(content);
+  }
+
+  /**
+   * Returns the length of the buffer.
+   */
+  int get length {
+    return _length;
+  }
+
+  bool isEmpty() {
+    return _length === 0;
+  }
+
+  /**
+   * Adds [obj] to the buffer. Returns [this].
+   */
+  StringBuffer add(Object obj) {
+    String str = obj.toString();
+    if (str === null || str.isEmpty()) {
+      return this;
+    }
+    _buffer.add(str);
+    _length += str.length;
+    return this;
+  }
+
+  /**
+   * Adds all items in [objects] to the buffer. Returns [this].
+   */
+  StringBuffer addAll(Collection objects) {
+    for (Object obj in objects) {
+      add(obj);
+    }
+    return this;
+  }
+
+  /**
+   * Adds the string representation of [charCode] to the buffer.
+   * Returns [this].
+   */
+  StringBuffer addCharCode(int charCode) {
+    return add(new String.fromCharCodes([charCode]));
+  }
+
+  /**
+   * Clears the string buffer. Returns [this].
+   */
+  StringBuffer clear() {
+    _buffer = new List<String>();
+    _length = 0;
+    return this;
+  }
+
+  /**
+   * Returns the contents of buffer as a concatenated string.
+   */
+  String toString() {
+    if (_buffer.length === 0) return "";
+    if (_buffer.length === 1) return _buffer[0];
+    String result = StringImplementation.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;
+  }
+
+  List<String> _buffer;
+  int _length;
+}