Pub is currently dogfooding the new Dart async/await syntax. Since the Dart VM doesn't natively support it yet, we are using the async-await compiler package.
We run that to compile pub-using-await from sdk/lib/_internal/pub down to vanilla Dart code which is what you see here. To interoperate more easily with the rest of the repositry, we check in that generated code.
When bug #104 is fixed, we can remove this entirely.
The code here was compiled using the async-await compiler at commit:
6228072d2a3fd87d6d4e8b147853c74486fdb581
(Note: this file is also parsed by a tool to update the above commit, so be careful not to reformat it.)