* Roll dart to c8f8c11b70.

Changes since last dart roll:
```
c8f8c11b70 Revert "[vm/fieldtable] Move current field values out of Field object into separate table."
746d3fb757 Use BoolExpressionVerifier in BinaryExpressionResolver.
7b44fa4265 Migrate "lib_2/developer" to NNBD.
c7745a1a0d contribute local completions for mixin members
d19b2b0349 Use NullableDereferenceVerifier in resolver instead of ErrorVerifier.
93b40dc037 NNBD preview: Flip non-null reasons back on
356fa11349 Extract BoolExpressionVerifier.
445c0842c7 Use nullable context type for operand of null-check operator. (2)
05632be0ce [nnbd/collection] fix infinite loop in nnbd sdk
8a947deffa [dartdevc] clear console on hot restart
```