Move Point and Rect over to dart:math, to be re-exported by dart:html.

tools/dom/templates/html/impl/impl_ClientRect.darttemplate

 // 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 $LIBRARYNAME;
 
 @DocsEditable()
-$(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS implements Rect$IMPLEMENTS$NATIVESPEC {
+$(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS implements Rectangle$IMPLEMENTS$NATIVESPEC {
 
-  // NOTE! All code below should be common with Rect.
-  // TODO(blois): implement with mixins when available.
-
+  // NOTE! All code below should be common with RectangleBase.
   String toString() {
     return '($left, $top, $width, $height)';
   }
 
   bool operator ==(other) {
     if (other is !Rect) return false;
     return left == other.left && top == other.top && width == other.width &&
         height == other.height;
   }
 
@@ skipping 72 matching lines @@
    * Truncates coordinates to integers and returns the result as a new
    * rectangle.
    */
   Rect toInt() => new Rect(left.toInt(), top.toInt(), width.toInt(),
       height.toInt());
 
   Point get topLeft => new Point(this.left, this.top);
   Point get bottomRight => new Point(this.left + this.width,
       this.top + this.height);
 $!MEMBERS}