* [dart] Improve types & imports
* don't use importMapping as it is intended for something different that is not possible in dart
* introduce imports map for dart specific features
* always import `dart:core`
* get rid of additionalReservedWords
* fix `--type-mappings` not working
* use required type mappings in samples
* no longer define additional reserved words as it is impossible to list all anyways, they can now be configured via type-mapping parameter
* simplify dio imports
* Don't use guava for map instantiation
* Update docs