The _internal construction is just a name often given to constructors that are private to the class (the name is not required to be ._internal you can create a private constructor using any Class._someName construction).
For example the following code only allows you to create new persons from outside the class using a caching constructor:
class Person {
final String name;
static Map<String,Person> _cache;
factory Person(String name) {
if(_cache === null) {
_cache = new Map<String,Person>();
}
if(_cache[name] === null]) {
_cache[name] = new Person._internal(name);
}
return _cache[name];
}
Person._internal(this.name);
}
In general Dart treats any _construction as private to either the class or the library that contains it. For example you can define as a global function like this:
_globalToThisLibaryOnly() {
print("can only be called globally within this #library");
}
which can be called from any file that is sourced within the library that defines it.