Missing feature: Reflectable does not yet handle `...` and `if` in constant collections

[eernstg]

This means that any use of reflectable that requires generation of code like, e.g., const [if (b) 42, ...[43, 44]] will fail. This issue is concerned with adding support for generating such code, or maybe expanding it into constructs which are supported today.

Note that it is still necessary to use --enable-experiment=constant-update-2018 to get support for these constructs in dart and dartanalyzer.