If tree shaker doesn't visit TypeParameterType.promotedBound which
contains the only reference to a particular class it may remove that
class, leaving a dangling reference from promotedBound. After that
kernel AST serialization would crash with "Missing canonical name for
Reference" error.
This change also fixes TypeParameterType.visitChildren.
TEST=pkg/vm/testcases/transformations/type_flow/transformer/regress_47878.dart
Fixes https://github.com/dart-lang/sdk/issues/47878
Change-Id: I555261713240bf0be412db27efb68f32cfd8008b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/222781
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Alexander Markov
1777f769ab