Fixing analyzer warnings on overloading == but not get hashCode.

tools/dom/src/Rectangle.dart

Index: tools/dom/src/Rectangle.dart
diff --git a/tools/dom/src/Rectangle.dart b/tools/dom/src/Rectangle.dart
index e87da1146b7cc1434b0a1a6569057750537ded52..e6a80b2f0edc67caa2607040ede9ad3f7d5653c8 100644
--- a/tools/dom/src/Rectangle.dart
+++ b/tools/dom/src/Rectangle.dart
@@ -54,6 +54,9 @@ class Rect {
         height == other.height;
   }
 
+  int get hashCode => JenkinsSmiHash.hash4(left.hashCode, top.hashCode,
+      width.hashCode, height.hashCode);
+
   /**
    * Computes the intersection of this rectangle and the rectangle parameter.
    * Returns null if there is no intersection.