Add dummy dart:html library for documentation.

lib/html/doc/interface/ValidityState.dartdoc

Index: lib/html/doc/interface/ValidityState.dartdoc
diff --git a/lib/html/doc/interface/ValidityState.dartdoc b/lib/html/doc/interface/ValidityState.dartdoc
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+// 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:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @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;
+}