Handle the unsigned shift operator in DDC

[floitschG]

This is the DDC issue for #30886.

This is a non-breaking change and is not required to be in Dart 2.0.
Probably blocked by #30889 .

[sigmundch]

I believe this will be mostly handled by the CFE and core libraries, so I mostly expect this issue to be validation that it all works properly.

[sigmundch]

@nshahan is this addressed by b250a58?

[nshahan]

Yes the implementation is in and this seems more or less done. I'll validate with the test suites and file bugs if there are any failures.