…tzf and dart data files

…n the tool/refresh.dart script

…mand description

…nd if the underlying utilities are not present

…ion with UTC by default

Replaced late-initialized fields with final fields initialized via a factory and private constructor.

…ions and overwrites

…tFile to avoid "dartimplementation_imports" lint error when consumed directly in other package

Import of a library in the 'lib/src' directory of another package.
Try importing a public library that exports this library, or removing the import.