As per planned breaking change to let platforms decide how they and what throw for late initiaization errors,
we no longer need a public `LateInitializationError` class. It's confusing to have one if some platforms throw
something else instead.
Removes the public abstract class. The dart:_internal implementation class `LateError` no longer implements it.
This is the only implementation of the public interface, and the class which platforms either throw directly,
or through front-end lowering of the feature.
Remove mentions in tests. All tests now just expect `Error`, some platform specific tests might test the message.
TEST=rewrote tests referring to LateInitializationError.
Change-Id: I54344a67f89ce101ed770412db134e12354cdcc4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/174928
Commit-Queue: Lasse R.H. Nielsen <lrn@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>