Add dummy dart:html library for documentation.

lib/html/doc/interface/SVGPathSegCurvetoCubicSmoothRel.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 SVGPathSegCurvetoCubicSmoothRel
+interface SVGPathSegCurvetoCubicSmoothRel extends SVGPathSeg {
+
+  /** @domName SVGPathSegCurvetoCubicSmoothRel.x */
+  num x;
+
+  /** @domName SVGPathSegCurvetoCubicSmoothRel.x2 */
+  num x2;
+
+  /** @domName SVGPathSegCurvetoCubicSmoothRel.y */
+  num y;
+
+  /** @domName SVGPathSegCurvetoCubicSmoothRel.y2 */
+  num y2;
+}