Tests async stacks for message channel API.
iframe created
breakpoint set before postMessage
paused at breakpoint
requested stepInto with breakOnAsyncCall flag
onmessage at :0:21
--postMessage--
(anonymous) at test.js:3:16

requested stepInto with breakOnAsyncCall flag
channel.port1.onmessage at :4:37
--postMessage--
onmessage at iframe.js:0:30
--postMessage--
(anonymous) at test.js:3:16