Add dummy dart:html library for documentation.

lib/html/doc/interface/SVGDefsElement.dart

Index: lib/html/doc/interface/SVGDefsElement.dart
diff --git a/lib/html/doc/interface/SVGDefsElement.dart b/lib/html/doc/interface/SVGDefsElement.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f65a3e4f67abf9f8290a17771cb27ca3f834e0f9
--- /dev/null
+++ b/lib/html/doc/interface/SVGDefsElement.dart
@@ -0,0 +1,9 @@
+// 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: Do not edit - generated code.
+
+/// @domName SVGDefsElement
+interface SVGDefsElement extends SVGElement, SVGTests, SVGLangSpace, SVGExternalResourcesRequired, SVGStylable, SVGTransformable {
+}