Don't suggest filing an issue when we can definitively say that the
assertion in question was not thrown from within the flutter package.
Fixes https://github.com/flutter/flutter/issues/3812.

Put the stack trace first after the message, with more details below the
stack trace, since the stack is often very useful and the stacks are no
longer stupidly long.

Better document the 'rescheduling' feature, and improve the error
handling asserts when it is misused.
Fixes https://github.com/flutter/flutter/issues/3888.

Improve the wording around the stack dump for exceptions during
callbacks.

Improve the color and font size of messages in live tests.
Fixes https://github.com/flutter/flutter/issues/4018.