## Description

This PRs changes the default value for the `platform` parameter used to simulate key events.

With this PR, the default value is "web" on web, otherwise it is the operating system name retrieved from `defaultTargetPlatform`.

Previously, for methods in `WidgetController`, it defaulted to “web” on web, and “android” everywhere else. And for methods in `KeyEventSimulator` it defaulted to “web” on web, and the operating system that the test was running on everywhere else. Because the operating system was based on `Platform.operatingSystem`, it usually differed from the target platform the test was running on.

AFAIK, the `platform` parameter is only meaningful for simulating `RawKeyEvent`. Once `RawKeyboard` will be fully removed, the `platform` parameter won’t be needed. 
@gspencergoog  In the meantime, do you think it is worth merging this fix?

## Related Issue

Fixes to https://github.com/flutter/flutter/issues/133955

## Tests

Adds one test.