Avoid leading underscores for library prefixesDRT-W1105

DON'T use a leading underscore for library prefixes. There is no concept of "private" for library prefixes. When one of those has a name that starts with an underscore, it sends a confusing signal to the reader. To avoid that, don't use leading underscores in those names.

BAD:

import 'dart:core' as _core;

GOOD:

import 'dart:core' as core;