Add dummy dart:html library for documentation.

lib/html/doc/interface/SVGPathSegMovetoRel.dartdoc

Index: lib/html/doc/interface/SVGPathSegMovetoRel.dartdoc
diff --git a/lib/html/doc/interface/SVGPathSegMovetoRel.dartdoc b/lib/html/doc/interface/SVGPathSegMovetoRel.dartdoc
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index 0000000000000000000000000000000000000000..07ab1c058625531898265928a7c3b1d5c78e2003
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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 SVGPathSegMovetoRel
+interface SVGPathSegMovetoRel extends SVGPathSeg {
+
+  /** @domName SVGPathSegMovetoRel.x */
+  num x;
+
+  /** @domName SVGPathSegMovetoRel.y */
+  num y;
+}