Previously, cases like:
```dart
Color.lerp(const Color(0xFF00FF7F), const Color(0xFF00FFFF), 1.1)
```
...would result in unexpected effects (in this instance, lerping
between these colors with a curve that overshoots would take what
should be a simple animation from pale green to blue and add some
flickering bright green whenever it overshoots).
* preserve Window callback zones
Run Window callbacks in the zone they are registered in. This is consistent with how other native API work, such as `scheduleMicrotask`, `Timer`, and `dart:io`. This also enables the developers to use the `Zone` API to capture and log unhandled Dart errors.
* refactor wrapping
* new line
* fewer if checks; group getters/setters/fields
* inline _invokeOnPointerDataPacket
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.
