Add all dart:html tests from the sdk to test/codegen.

test/codegen/lib/html/client_rect_test.dart

+library ClientRectTest;
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+import 'dart:html';
+
+main() {
+
+  var isRectList =
+      predicate((x) => x is List<Rectangle>, 'is a List<Rectangle>');
+
+  insertTestDiv() {
+    var element = new Element.tag('div');
+    element.innerHtml = r'''
+    A large block of text should go here. Click this
+    block of text multiple times to see each line
+    highlight with every click of the mouse button.
+    ''';
+    document.body.append(element);
+    return element;
+  }
+
+  useHtmlConfiguration();
+
+   test("ClientRectList test", () {
+    insertTestDiv();
+    var range = new Range();
+    var rects = range.getClientRects();
+    expect(rects, isRectList);
+  });
+}