[cfe] Use unchecked AsExpression instead of promoted VariableGet

This changes the pattern lowering to use unchecked AsExpression
instead of promoted VariableGet when this occurs within a late
cache variable initializer. This is done to avoid promoted variables
that violate the (otherwise used) conservative reasoning about the
execution order of closures.

The unchecked AsExpressions are now shown in the ast-to-text.

Change-Id: I4fa6813f3b5cc73d9cdac17054de59e5d4b8ce7d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/307044
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Nate Biggs <natebiggs@google.com>