Support assert in initializers in the analyzer

[bwilkerson]

The full syntax should already be supported, but not all error messages are being generated. Start by determining which error messages need to be generated and then implement them.

There are also potential implications for constant evaluation that need to be looked at.

[scheglov]

Language issue: #30968
Informal specification: https://github.com/dart-lang/sdk/blob/master/docs/language/informal/assert-in-initializer-list.md

[bwilkerson]

Two CL's in flight:
add evaluation support: https://dart-review.googlesource.com/c/sdk/+/32100.
add check for non-bool expressions: https://dart-review.googlesource.com/c/sdk/+/31963.

[bwilkerson]

Both have now landed, so I believe this arc of work is complete.