Dart2js: Assert in initializer list.

[lrhn]

We have decided to make asserts in initializer lists part of the language. Dart2js needs to implement the feature.
See: docs/language/informal/assert-in-initializer-list.md for a description of the feature.

[sigmundch]

We'll implement this after we migrate to use the shared FE, but we will keep the old pipeline as is for the time being.

[rakudrama]

We should hold off on implementing the feature until Kernel has defined what it looks like.
#30914

[dgrove]

Any updates on this, @sigmundch ?

[sigmundch]

This is already implemented.