Add dummy dart:html library for documentation.

lib/html/doc/interface/SVGLineElement.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 SVGLineElement
+interface SVGLineElement extends SVGElement, SVGTests, SVGLangSpace, SVGExternalResourcesRequired, SVGStylable, SVGTransformable {
+
+  /** @domName SVGLineElement.x1 */
+  final SVGAnimatedLength x1;
+
+  /** @domName SVGLineElement.x2 */
+  final SVGAnimatedLength x2;
+
+  /** @domName SVGLineElement.y1 */
+  final SVGAnimatedLength y1;
+
+  /** @domName SVGLineElement.y2 */
+  final SVGAnimatedLength y2;
+}