Add dummy dart:html library for documentation.

lib/html/doc/interface/SVGLocatable.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 SVGLocatable
+interface SVGLocatable {
+
+  /** @domName SVGLocatable.farthestViewportElement */
+  final SVGElement farthestViewportElement;
+
+  /** @domName SVGLocatable.nearestViewportElement */
+  final SVGElement nearestViewportElement;
+
+  /** @domName SVGLocatable.getBBox */
+  SVGRect getBBox();
+
+  /** @domName SVGLocatable.getCTM */
+  SVGMatrix getCTM();
+
+  /** @domName SVGLocatable.getScreenCTM */
+  SVGMatrix getScreenCTM();
+
+  /** @domName SVGLocatable.getTransformToElement */
+  SVGMatrix getTransformToElement(SVGElement element);
+}