[ddc] Use `.futureValueType` from CFE
[ddc] Use `.futureValueType` from CFE

The CFE FunctionNode has a new getter for the Future value type that
can be used in null safe libraries.

Legacy libraries will still rely on flatten to get the type of the
Future.

Change-Id: I54ad3bc096fdb981f7f499f2ba3ddfcb01fd97d6
Issue: https://github.com/dart-lang/sdk/issues/44745
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/185240
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>
Commit-Queue: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@google.com>

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