Build-in identifiers are permitted as prefix.

[ngolovleva]

According to Dart Language specification (4th edition, Dec 2015) "16.33 Identifier Reference":

"It is a compile-time error if a built-in identifier is used as the declared name of a prefix, class, type parameter or type alias."

However, dart compiles successfully test below.

library lib;
var x = 1;

import "lib.dart" as abstract;
main() {
  abstract.x;
}

The same is true for other build-in identifiers: as, deferred, dynamic, export, external, factory, get, implements, import, library, operator, part, set, static, typedef.

[floitschG]

Dart2js and analyzer don't complain either.
I'm not sure, if there are corner-cases where a prefix with a built-in identifier would hurt.