This reverts commit 0b6b3eec.

df39e5c51 Roll Dart SDK from d75741b19133 to a92baed5f8e2 (1 revision) (flutter/engine#22944) (#71969)

Element.rebuild() and BuildOwner.buildScope() both had code to
manager the `BuildOwner._debugCurrentBuildTarget` state variable.
Unfortunately, only one of the two places was using a try/finally
to ensure that the state variable was restored even if an error
was thrown.

This creates a private method to unify the copy/pasted code.

This reverts commit 70f8fdee.

Fix the duration assertion error for the _animateToInternal method and refine the error description (#71137)

* StandardMethodCodec.decodeEnvelope should allow null error message