Fix up static analysis errors introduced by Futures CL.

tools/dom/templates/html/impl/impl_RTCPeerConnection.darttemplate

 // Copyright (c) 2013, 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.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $if DART2JS
   factory $CLASSNAME(Map rtcIceServers, [Map mediaConstraints]) {
     var constructorName = JS('RtcPeerConnection', 'window[#]',
@@ skipping 21 matching lines @@
     // is supported at at the given time.
     try {
       var c = new RtcPeerConnection({"iceServers": [ {"url":"stun:foo.com"}]});
       return c is RtcPeerConnection;
     } catch (_) {}
     return false;
   }
 $else
   static bool get supported => true;
 $endif
+  Future<RtcSessionDescription> createOffer([Map mediaConstraints]) {
+    var completer = new Completer<RtcSessionDescription>();
+    _createOffer(
+        (value) { completer.complete(value); },
+        (error) { completer.completeError(error); }, mediaConstraints);
+    return completer.future;
+  }
+
+  Future<RtcSessionDescription> createAnswer([Map mediaConstraints]) {
+    var completer = new Completer<RtcSessionDescription>();
+    _createAnswer(
+        (value) { completer.complete(value); },
+        (error) { completer.completeError(error); }, mediaConstraints);
+    return completer.future;
+  }
 $!MEMBERS
 }