[ddc] Inline and remove cast from `unsafeCast()`
[ddc] Inline and remove cast from `unsafeCast()`

Avoids an extra function call and removes a casts that are
always expected to succeed from the compiled code in the
sdk async library. The motivating changes are introduced in
https://dart-review.googlesource.com/c/sdk/+/230240.

Change-Id: I4da07a54f025ee4e2d0a5a0b4fd4ce9c7e765454
Issue: https://github.com/dart-lang/sdk/issues/48225
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/230884
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>
Reviewed-by: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>
Commit-Queue: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@google.com>

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