* Avoid setTimeout in Node, because we don't need the overhead there.
* Still call setTimeout in browsers to prevent the tab from being killed.
* Use queueMicrotask in Safari, because it's dramatically faster than
MessageChannel there.
* Continue to use MessageChannel in other supported browsers becuase it's
somewhat faster than queueMicrotask there.
* Don't use setImmediate any more because there's a faster alternative in
all supported envs.
In jasmine-core's own test suite, this yields a roughly 50-70% speedup
in Node, ~20% in Edge, and 75-90%(!) in Safari.
