Add dummy dart:html library for documentation.

lib/html/doc/interface/SessionDescription.dartdoc

Index: lib/html/doc/interface/SessionDescription.dartdoc
diff --git a/lib/html/doc/interface/SessionDescription.dartdoc b/lib/html/doc/interface/SessionDescription.dartdoc
new file mode 100644
index 0000000000000000000000000000000000000000..af8e98f8a29c9b447b19dab09a79535cb2e3862f
--- /dev/null
+++ b/lib/html/doc/interface/SessionDescription.dartdoc
@@ -0,0 +1,19 @@
+// 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 SessionDescription
+interface SessionDescription default _SessionDescriptionFactoryProvider {
+
+  SessionDescription(String sdp);
+
+  /** @domName SessionDescription.addCandidate */
+  void addCandidate(IceCandidate candidate);
+
+  /** @domName SessionDescription.toSdp */
+  String toSdp();
+}