Add dummy dart:html library for documentation.

lib/html/doc/interface/ValidityState.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.
+
+// WARNING: Do not edit - generated code.
+
+/// @domName ValidityState
+interface ValidityState {
+
+  /** @domName ValidityState.customError */
+  final bool customError;
+
+  /** @domName ValidityState.patternMismatch */
+  final bool patternMismatch;
+
+  /** @domName ValidityState.rangeOverflow */
+  final bool rangeOverflow;
+
+  /** @domName ValidityState.rangeUnderflow */
+  final bool rangeUnderflow;
+
+  /** @domName ValidityState.stepMismatch */
+  final bool stepMismatch;
+
+  /** @domName ValidityState.tooLong */
+  final bool tooLong;
+
+  /** @domName ValidityState.typeMismatch */
+  final bool typeMismatch;
+
+  /** @domName ValidityState.valid */
+  final bool valid;
+
+  /** @domName ValidityState.valueMissing */
+  final bool valueMissing;
+}