Add test for DOM fileapi.

tests/html/fileapi_test.dart

+#library('fileapi');
+#import('../../lib/unittest/unittest.dart');
+#import('../../lib/unittest/html_config.dart');
+#import('dart:html');
+
+main() {
+  useHtmlConfiguration();
+  window.webkitRequestFileSystem(Window.TEMPORARY, 100, (fs) {
+    // FIXME: add types to callback arguments after migration to wrapperless dart:html.
+    asyncTest('getDirectory', 2, () {
+      Expect.throws(() => fs.root.getDirectory('directory1', {'x': true}, (e) {}));
+
+      fs.root.getDirectory(
+          'directory2',
+          flags: {},
+          successCallback: (e) {
+            Expect.fail('Should not be reached');
+            callbackDone();
+          },
+          errorCallback: (e) {
+            Expect.equals(FileError.NOT_FOUND_ERR, e.code);
+            callbackDone();
+          });
+
+      fs.root.getDirectory(
+          'directory3',
+          flags: {'create': true},
+          successCallback: (e) {
+            Expect.equals('directory3', e.name);
+            callbackDone();
+          },
+          errorCallback: (e) {
+            Expect.fail('Got file error: ${e.code}');
+            callbackDone();
+          });
+    });
+
+    asyncTest('getFile', 2, () {
+      Expect.throws(() => fs.root.getFile('file1', {'x': true}, (e) {}));
+
+      fs.root.getDirectory(
+          'file2',
+          flags: {},
+          successCallback: (e) {
+            Expect.fail('Should not be reached');
+            callbackDone();
+          },
+          errorCallback: (e) {
+            Expect.equals(FileError.NOT_FOUND_ERR, e.code);
+            callbackDone();
+          });
+
+      fs.root.getDirectory(
+          'file3',
+          flags: {'create': true},
+          successCallback: (e) {
+            Expect.equals('file3', e.name);
+            callbackDone();
+          },
+          errorCallback: (e) {
+            Expect.fail('Got file error: ${e.code}');
+            callbackDone();
+          });
+    });
+  });
+}