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

tests/html/rect_test.dart

Index: tests/html/rect_test.dart
diff --git a/tests/html/rect_test.dart b/tests/html/rect_test.dart
index fd1935a2485b4c5f4f19cc8313014091768f6f3a..8b52f615839af2695559138224849a928e541430 100644
--- a/tests/html/rect_test.dart
+++ b/tests/html/rect_test.dart
@@ -147,4 +147,13 @@ main() {
     expect(b.width is int, isTrue);
     expect(b.height is int, isTrue);
   });
+
+  test('hashCode', () {
+    var a = new Rect(0, 1, 2, 3);
+    var b = new Rect(0, 1, 2, 3);
+    expect(a.hashCode, b.hashCode);
+
+    var c = new Rect(1, 0, 2, 3);
+    expect(a.hashCode == c.hashCode, isFalse);
+  });
 }