Two years ago when the tables were turned and Google announced Dart, which has the same compatibility strategy as asm.js (fastest with explicit support, falls back to regular JS), Brendan Eich had these harsh words for Google's move (https://news.ycombinator.com/item?id=2982949):

    So "Works best in Chrome" and even "Works only in Chrome"
    are new norms promulgated intentionally by Google. We see
    more of this fragmentation every day. As a user of Chrome
    and Firefox (and Safari), I find it painful to experience,
    never mind the political bad taste.

But now "runs best in Mozilla Firefox" is cool?

I actually like asm.js and think it's a totally reasonable approach. I mention this only to highlight that maybe all of the bad-faith assumptions Google gets are not entirely deserved. Objectively these two are similar moves and I think they are both in good faith.