Delete file loading via mmap

[werediver]

Delete file loading via mmap(), because mmap() doesn't seem to provide any benefits for the use-cases of this project.

Some of the arguments:

• Majority of Dart source files I could check (including Flutter SDK and a large mobile app project) are under 4 KiB in size, which means even mmap() would just load them completely on the first access
• Optimizing for the "good weather" scenario (a successful parse), UTF-8 validation of the source text should be done for the entire file (and preferably in one go, because the validation code doesn't seem very compact, so interleaving it with other complicated routines may just cause inefficient execution)