In some formats variables are declared as `var` or `let` and only
assigned a value later in the code.
The ngtsc resolver still needs to be able to resolve this value,
so the host now provides a `host.getVariableValue(declaration)`
method that can do this resolution based on the format.
The hosts make some assumptions about the layout of the
code, so they may only work in the constrained scenarios that
ngcc expects.
PR Close#26236
Pete Bacon Darwin