In order to support the text parameter, we need to synthesize an Input event independently of KeyboardEvents. In other words, the value text is what should show up in an input field, regardless of what value is provided by KeyboardEvent.key.

In puppeteer running with Chrome:

await page.keyboard.down("KeyA", {"text": "L"});
await page.keyboard.up("KeyA");

The corresponds to CDP commands: (note that "text" is L)

{"method":"Input.dispatchKeyEvent","params":{"type":"keyDown","modifiers":0,"windowsVirtualKeyCode":65,"code":"KeyA","key":"a","text":"L","unmodifiedText":"L","autoRepeat":false,"location":0,"isKeypad":false}}
{"method":"Input.dispatchKeyEvent","params":{"type":"keyUp","modifiers":0,"key":"a","windowsVirtualKeyCode":65,"code":"KeyA","location":0}} 

keyboard events: note that the keyboard event has 'a', but the input event has 'L' and 'L' is displayed in the input element.

[ { code: 'KeyA',
    key: 'a',
    which: 65,
    location: 0,
    ctrl: false,
    meta: false,
    shift: false,
    repeat: false,
    type: 'keydown' },
  { code: 'KeyA',
    key: 'a',
    which: 76,
    location: 0,
    ctrl: false,
    meta: false,
    shift: false,
    repeat: false,
    type: 'keypress' },
  { data: 'L', inputType: 'insertText', isComposing: false },
  { code: 'KeyA',
    key: 'a',
    which: 65,
    location: 0,
    ctrl: false,
    meta: false,
    shift: false,
    repeat: false,
    type: 'keyup' } ]

As far as I can tell, there's no way to synthesize a trusted Input event via EventUtils or TextInputProcessor such that it represents different text data than the corresponding KeyboardEvents.

This isn't used in the gutenberg test suite and is a relatively esoteric use-case so I don't think it needs to be in alpha.