DartPad inject script sometimes fails

[johnpryan]

When I was testing dart-lang/site-www#5627 (comment) I noticed that some of the DartPad embeds are displaying the default snippet. We might need to listen for these events right away, instead of when the Editor widget is created (#2858)

[parlough]

Taking a look at this, I think it's perhaps not listening to the event too late, but rather not distinguishing between the embeds on the page for which sent the "ready" message. I'll investigate adding a way to distinguish between them.