[dart2js] Implement support for generic function instantiation of existing function objects in constants

[eernstg]

This is the dart2js specific issue for #47150, which has the details.

[sigmundch]

based on the earlier discussions, here too I believe this is a noop for dart2js

[fishythefish]

I agree, but I'll leave this open until the CFE implementation lands. Also passing this to @rakudrama, who has been handling other instantiation-related work.

[devoncarew]

FYI, the related CFE issue - #47154 - is closed.

[fishythefish]

Closing since #47154 is closed and language/const/instantiated_function_constant_error_test is passing.