`@docImport`s should support `as` imports

[goderbauer]

Something like this should work:

/// @docImport 'dart:io' as io;
library;

/// You can use [io.Stdin] to...
void foo() {
  // intentionally left blank
}

[dart-github-bot]

Summary: The @docImport annotation currently doesn't support the as keyword for aliasing imported libraries, making it difficult to reference imported symbols in documentation comments. This issue proposes adding support for as imports within @docImport annotations.