Throw exceptions rather than crashing when Canvas API is misused. (fl…

…utter#3811)

This attempts to make the Canvas API and some related features more
likely to throw a Dart exception than crash when exposed to bad input.

Depends on rolling tonic to
https://fuchsia-review.googlesource.com/c/35742/ which this patch does
not yet do, but I wanted to put it up for review to see if it was even
a reasonable approach.