Marks `Function` as an extensible type so that DDC knows to use the
symbolized version of the equals operator.
The `Function` class at runtime already has its equals method attached
as the "symbolized" version. Consider `a == b`. When DDC knows `a` is
statically a raw `Function` and non-nullable (more likely with sound
null safety) the generated code should call the symbolized equals
member ex: `a[$_equals](b)`. Without this change the generated code
would be `a.equals(b)` and fail at runtime because the method does
not exist.
With this change co19_2/LibTest/core/Function/operator_eq_A01_t01
starts passing.
Change-Id: I80dd2abbbb04f1b7ab7e21dd14561a45f6e81459
Fixes: https://github.com/dart-lang/sdk/issues/45601
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/194204
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Konstantin Shcheglov