This standardizes the handling of nullability for the LogicalKeyboardKey.keyLabel and RawKeyEventData.keyLabel accessors so that they are non-nullable, but can be empty.

Before this change, the keyLabel could be either null or an empty string to indicate that there wasn't a label, which makes it harder to test for, since both need to be checked for. Since an empty string is sufficient, there is no need for it to be nullable.

Also, in raw_keyboard.dart, the web and Windows implementations wouldn't accept null values for parameters in the Map coming from the message, but tests were supplying null for some of them. This makes web and Windows creation of events match the other platforms, and makes the migration of tests to non-nullability easier.

Like Android/iOS, only enable --null-assertions if asked. Previously this was enabled by default for web, but in general this has proved to be too breaking to enable by default.

#61042

Attempt to simplify the Android SDK interface ahead of refactoring it. The locateAndroidSdk static method is called at startup to locate the android SDK, returning null if it cannot be found. These helper methods attempted to first look up the AndroidSDK if it was already null - which could only cover the case where someone installed the Android SDK while flutter was running (possibly through an IDE)

Reland of acdb909f with fixes to semantics.dart

#61042

These tests look quite stable for the last 10 runs.

Bug:
  https://github.com/flutter/flutter/issues/64121

This reverts commit acdb909f.

Focus will be moved automatically if onEditingComplete is not specified, but must
by moved manually if onEditingComplete is specified.

This reverts commit 1de673bc.

Revert "Prevent viewport.showOnScreen from scrolling the viewport if the specified Rect is already visible.  (#56413)" (#64091)

This reverts commit 64d76f2f.

Also output covered | total lines, and a summary percentage at the end + Kick tree

This reverts commit 72619b86.