Don't pin package:macros

[christopherfujino]

Full context #148004

TL;DR

1. Because package:macros depends on package:_macros via a Dart SDK dependency; and
2. package:macros exactly pins package:_macros; and
3. Each new version of package:macros will increment its exact pin of package:_macros (comment); and
4. The flutter/flutter repository exactly pins the Dart SDK (via its exact pin on the flutter/engine repository

Therefore, the flutter/flutter repository effectively pins both package:macros and package:_macros already (as in, there exists only a single version of each that pub will successfully be able to solve within the context of a particular Flutter SDK).

Therefore, it is safe for flutter/flutter repository to not pin package:macros, which will allow engine -> framework rolls that contain a new Dart SDK that contains a new package:_macros version to land automatically, provided all tests pass.

[christopherfujino]

For science, I pushed the commit 58a3acd, which rolls the engine to an older roll that failed CI in #148008, because pub was not able to solve successfully with the exact pins of package:macros. If CI passes on that commit, I will revert it and land this change without it, and then re-enable the dart -> engine roller (FYI @rmacnak-google)

[christopherfujino]

For science, I pushed the commit 58a3acd, which rolls the engine to an older roll that failed CI in #148008, because pub was not able to solve successfully with the exact pins of package:macros. If CI passes on that commit, I will revert it and land this change without it, and then re-enable the dart -> engine roller (FYI @rmacnak-google)

All presubmits passed except for two integration test flakes unrelated to pub solving package:macros, so I will call this a successful test: https://github.com/flutter/flutter/pull/148087/checks?check_run_id=24794486500

Reverted the engine revision change in 17d5dc5

[christopherfujino]

Merging manually because autosubmit bot is broken: #148092