Previously, `uninitialized()` was a method, requiring a call as well as
two extra characters everywhere it was used.
Make this value a variable, saving the characters and avoiding the
method call to get its value.
This change also removes the export of `uninitialized` from
change_detect.ts, which is technically a breaking change, however
`uninitialized` is an implementation detail and nobody should be using
it in app logic. By convention, apps should not be importing from files
under `src/`.
Update to #3248.
03fc7fe8c2