Adds support for layout tests.

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 div1Style = _style('blue', 20, 10, 40, 10);
+  var div2Style = _style('red', 25, 30, 40, 10);
+  var div1 = new Element.html('<div style="$div1Style"></div>');
+  var div2 = new Element.html('<div style="$div2Style"></div>');
+  document.body.elements.add(div1);
+  document.body.elements.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;');
+}