This adds an assert in RawKeyboard that catches the case where it tries to send a key down event, but (after synchronizing modifiers) there are no keys in keysPressed. This state can occur if the modifier flags are not set properly for the platform.

Also prevents shortcuts attempting to handle a key down when no keys are pressed at the moment (which was causing a crash in release mode).

It now uses the scroll metrics as they stood at the end of the last frame.

It previously used a weird combination of the old extents and the newish position, which led to some weird effects when the position had been changed in expectation of a viewport or content dimension change.

Framework tests got replaced with a new version using shards. This pr is
enabling the new builders in presubmit and removing the old ones.

Bug:
  https://github.com/flutter/flutter/issues/63258
  https://github.com/flutter/flutter/issues/63242
  https://github.com/flutter/flutter/issues/63241

* Ignore dead_code hints for weak-only null checks.

When https://github.com/dart-lang/sdk/issues/41985 is fixed, the
analyzer will begin detecting dead code due to "unnecessary" null
checks.  Since we want to retain null checks in Flutter even when they
appear unnecessary (in order to preserve runtime behavior in weak
mode), we need to suppress these dead code hints.

* Add comments explaining why we're ignoring dead_code hints

This reverts commit 74b5051c.

* Add native stacktrace field for PlatformException

* Mute the readValue check for stacktrace.

* polish

* Add unit test and further polish

* Added more comments

* remove unnecessary import

* fill in stacktrace to JSONMethodCodec and fix tests
Co-authored-by: Ben Li <libe@google.com>