Copy lib/dom tests that use dart:html to lib/html.

tests/dom/xhr_test.dart

-#library('XHRTest');
-#import('../../lib/unittest/unittest.dart');
-#import('../../lib/unittest/html_config.dart');
-#import('dart:html');
-
-main() {
-  useHtmlConfiguration();
-  asyncTest('XHR', 1, () {
-    XMLHttpRequest xhr = new XMLHttpRequest();
-    // TODO: figure out how to place a resource file with fixed name alongside
-    // the .html file.
-    xhr.open('GET', 'MISSING-FILE', true);
-    xhr.on.readyStateChange.add((event) {
-        if (xhr.readyState == 4) {
-          Expect.equals(0, xhr.status);
-          Expect.stringEquals('', xhr.responseText);
-          callbackDone();
-        }
-      }, true);
-    xhr.send();
-  });
-}