Whether the current canonicalize invocation comes from an
@import rules, URLs should canonicalize to an
import-only file if one exists for the URL being canonicalized.
Otherwise, canonicalization should be identical for
Subclasses should only access this from within calls to canonicalize. Outside of that context, its value is undefined and subject to change.
@protected @nonVirtual bool get fromImport => utils.fromImport;