Rather that importing `package:newton/newton.dart` you can
`import package:flutter/physics.dart`.

Fixes #2441

We were setting a negative velocity tolerance, which never triggered because we
assume tolerances are positive numbers. Now we use a positive tolerance and
assert to catch this case in the future.

Fixes #2765

Fixes #1355

We don't need to compute simulations out to that many decimal places because
we're working in pixels.

R=chinmaygarde@google.com