Sometimes the type expression of a is-test can be widened to one that
does not use type variables:
```dart
void addAll(Iterable<E> items) {
if (items is List<E>) {
... items[i] ... // code specialized to lists
}
...
}
```
Changing `is List<E>` to `is List` is more efficient, but not possible
at the source level since the explicit type parameter is needed for
type promotion.
This change uses a predicate provided by the Kernel package to widen
an interface type when generating the CFG instructions for the
is-test. This can lead to knock-on optimizations like compiling `is X`
to `instanceof`.
Bug: #54998
Change-Id: I956b9d9b8a31ae40aee86e31def475baa9ab5cfe
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/408124
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Stephen Adams <sra@google.com>
Stephen Adams
e962350dd6