Add dummy dart:html library for documentation.

lib/html/doc/interface/SVGMaskElement.dartdoc

+// 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 SVGMaskElement
+interface SVGMaskElement extends SVGElement, SVGTests, SVGLangSpace, SVGExternalResourcesRequired, SVGStylable {
+
+  /** @domName SVGMaskElement.height */
+  final SVGAnimatedLength height;
+
+  /** @domName SVGMaskElement.maskContentUnits */
+  final SVGAnimatedEnumeration maskContentUnits;
+
+  /** @domName SVGMaskElement.maskUnits */
+  final SVGAnimatedEnumeration maskUnits;
+
+  /** @domName SVGMaskElement.width */
+  final SVGAnimatedLength width;
+
+  /** @domName SVGMaskElement.x */
+  final SVGAnimatedLength x;
+
+  /** @domName SVGMaskElement.y */
+  final SVGAnimatedLength y;
+}