[web] Hides that Flutter uses requireJS in debug. (#129032)
Flutter web uses requireJS in `debug` mode to assemble a DDC-compiled app from a bunch of small files ("modules").
This caused that `canvaskit.js` (and all other modules that used a browserify-like loading header) didn't work because they attempted to use the `define` function provided by Flutter's instance of `requireJS` (which kept the defined modules private, rather than as globals on the page, as the users of the JS expected).
A [fix](https://github.com/flutter/engine/pull/27342) was added to `flutter/engine` to trick loaders into *not* using the `requireJS` module loader, but a recent change in the fix's js-interop layer *subtly* changed its JS output on the page (objects went from `undefined` to `null`), causing this:
* https://github.com/flutter/flutter/issues/126131 (and others)
This PR hides a bit of code that is commonly used by module loaders to decide that they may use the `define` function provided by requireJS (so the engine workaround can be removed).
## Next steps
* https://github.com/flutter/engine/pull/42941
## Issues
Partially addresses: https://github.com/flutter/flutter/issues/126131 (and others)
## Tests
* Added a unit test to ensure the `delete` stays
* Manually tested with the Gallery app in `debug` mode with a bunch of user-supplied scripts that currently fail to load.
* Also tested hot restart as suggested by @nshahan
https://dart.googlesource.com/external/github.com/flutter/flutter/+/34b42acf1b258f0cc1330bf27a15d96c10098eee
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