Previously, the behaviour of lerpDouble with respect to NaN and infinity
was relatively complex and difficult to reason about. This patch
simplifies the behaviour with respect to those conditions and adds
documentation and tests.

In general, if `a == b` or both values are null, infinite, or NaN, `a`
is returned. Otherwise we require `a` and `b` and `t` to be finite or
null and the result of the linear interpolation is returned.

This extracts a Dart test utilities library, containing
`expectAssertion` and `expectArgumentError` functions that simplify
running tests that test assertions across debug, profile, and release
configurations.

This change also restricts Dart unit tests to testing files whose
filename matches `*_test.dart` under `flutter/testing/dart`; previously
any file in that directory was run, but all files matched the above
pattern.

The `a` and `b` parameters to `lerpDouble` have type `num`. This adds
tests for integer parameter values.

Adds tests for lerpDouble for cases where NaN or infinite values are
passed. These tests simply specify the current behaviour.

This re-lands the key event synthesis implementation for Android (Original PR: #19024, Revert PR: #19956). The only difference is sending the synthesized key events to the root view instead of the current view.

Without sending it to the root view, the system doesn't have any chance of handling keys like the back button. The event will still not be sent to the framework twice, since we turn off event propagation while re-dispatching the event.

* Switched PathCommand apply parameter to non-nullable. Add clip op parameter to ClipCommand.