evaluator: required fields are dropped by cue eval

[rogpeppe]

What version of CUE are you using (cue version)?

0b681f5

Does this issue reproduce with the latest stable release?

N/A

What did you do?

exec cue eval x.cue
cmp stdout want.cue
-- x.cue --
a: x!: 5
-- want.cue --
a: x!: 5

What did you expect to see?

Passing test.

What did you see instead?

> exec cue eval x.cue
[stdout]
a: {}
> cmp stdout want.cue
--- stdout
+++ want.cue
@@ -1,1 +0,0 @@
-a: {}
@@ -0,0 +1,1 @@
+a: x!: 5

FAIL: /tmp/testscript4012701096/x.txtar/script.txtar:2: stdout and want.cue differ