Provide cross-browser Rects for box model dimensions for Elements.

tools/dom/src/Rectangle.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 class for representing two-dimensional rectangles.
+ * A base class for representing two-dimensional rectangles. This will hopefully
+ * be moved merged with the dart:math Rect.
  */
-class Rect {
-  final num left;
-  final num top;
-  final num width;
-  final num height;
-
-  const Rect(this.left, this.top, this.width, this.height);
-
-  factory Rect.fromPoints(Point a, Point b) {
-    var left;
-    var width;
-    if (a.x < b.x) {
-      left = a.x;
-      width = b.x - left;
-    } else {
-      left = b.x;
-      width = a.x - left;
-    }
-    var top;
-    var height;
-    if (a.y < b.y) {
-      top = a.y;
-      height = b.y - top;
-    } else {
-      top = b.y;
-      height = a.y - top;
-    }
-
-    return new Rect(left, top, width, height);
-  }
+// TODO(efortuna): Merge with Math rect after finalizing with Florian.
+abstract class RectBase {
+  num get left;
+  num get top;
+  num get width;
+  num get height;
 
   num get right => left + width;
   num get bottom => top + height;
 
   // NOTE! All code below should be common with Rect.
-  // TODO: implement with mixins when available.
 
   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 73 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);
 }
+
+
+
+/**
+ * A class for representing two-dimensional rectangles.
+ *
+ * This class is distinctive from RectBase in that it enforces that its
+ * properties are immutable.
+ */
+class Rect extends RectBase {
+  final num left;
+  final num top;
+  final num width;
+  final num height;
+
+  const Rect(this.left, this.top, this.width, this.height);
+
+  factory Rect.fromPoints(Point a, Point b) {
+    var left;
+    var width;
+    if (a.x < b.x) {
+      left = a.x;
+      width = b.x - left;
+    } else {
+      left = b.x;
+      width = a.x - left;
+    }
+    var top;
+    var height;
+    if (a.y < b.y) {
+      top = a.y;
+      height = b.y - top;
+    } else {
+      top = b.y;
+      height = a.y - top;
+    }
+
+    return new Rect(left, top, width, height);
+  }
+}