Assert for RenderFlex intrinsics if using baseline alignment

[goderbauer]

Description

When CrossAxisAlignment.baseline is used in a RenderFlex we can't actually calculate any intrinsics sizes because baseline metrics are only available after layout (and the intrinsics are explicitly not doing a full layout). As documented for computeMinIntrinsicWidth under "When the intrinsic dimensions cannot be known", in these cases we should throw. Unfortunately, upto now that's not what we do. This PR implements the documented contract.

We do something similar for RenderParagraph in its _canComputeIntrinsics method.

Related Issues

#65895

Tests

I added the following tests:

• added a test for RenderFlex

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[goderbauer]

/cc @tvolkert @Hixie

[tvolkert]

LGTM