Create `dart:mirrors` stubs for `dart2js` and `DDC`

[bkonyi]

[lrhn]

The most forwards compatible way to do this is to move all VM implementation into a patch file, and then add throwing implementations as a patch file for the web compilers.

That ensures that if any API gets added or changed in the future, the web patch files will also be updated, where a separate mock library might be forgotten.

(That's probably not an issue for dart:mirrors, which is unlikely to ever change again, other than hopefully going away entirely, but it may be more relevant for other libraries.)

Why is anyone importing dart:mirrors in a program that they want to compile for the web. IIRC the mirrors library isn't available in Flutter, and not even in AoT VM compilation. Making it importable in AoT web seems like going further than the VM itself does.