Remove pixel/layout tests in tests/utils + special casing in the testing scripts for pixel/layout t…

tests/utils/png_layout_test.dart

-// Copyright (c) 2012, 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.
-
-/**
- * Tests the layout test functionality of test.dart. There is a .png image with
- * the same name as this file. The existence of the .png file indicates to the
- * test framework that we want to compare the final state of this application
- * against an image file.
- */
-library layouttest;
-
-import 'dart:html';
-
-main() {
-  var div1 = new DivElement();
-  div1.attributes['style'] = _style('blue', 20, 10, 40, 10);
-  var div2 = new DivElement();
-  div2.attributes['style'] = _style('red', 25, 30, 40, 10);
-  document.body.children.add(div1);
-  document.body.children.add(div2);
-}
-
-_style(String color, int top, int left, int width, int height) {
-  return ('background-color:$color; position:absolute; '
-          'top:${top}px; left:${left}px; '
-          'width:${width}px; height:${height}px;');
-}