Make Rectangle and MutableRectangle constructors handle negative lengths.

tools/dom/src/CssRectangle.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 html;
 
 /**
  * A rectangle representing all the content of the element in the
  * [box model](http://www.w3.org/TR/CSS2/box.html).
  */
@@ skipping 139 matching lines @@
  *
  * In contrast to the more general purpose [Rectangle] class, this class's
  * values are mutable, so one can change the height of an element
  * programmatically.
  *
  * _Important_ _note_: use of these methods will perform CSS calculations that
  * can trigger a browser reflow. Therefore, use of these properties _during_ an
  * animation frame is discouraged. See also:
  * [Browser Reflow](https://developers.google.com/speed/articles/reflow)
  */
-abstract class CssRect extends MutableRectangle<num> implements Rectangle<num> {
+abstract class CssRect extends MutableRectangle<num> {
   Element _element;
 
   CssRect(this._element) : super(0, 0, 0, 0);
 
   num get left;
 
   num get top;
 
   /**
    * The height of this rectangle.
@@ skipping 82 matching lines @@
     }
     return val;
   }
 }
 
 final _HEIGHT = ['top', 'bottom'];
 final _WIDTH = ['right', 'left'];
 final _CONTENT = 'content';
 final _PADDING = 'padding';
 final _MARGIN = 'margin';