Don't pin package:macros (#148087)
Full context https://github.com/flutter/flutter/issues/148004
TL;DR
1. Because `package:macros` depends on `package:_macros` via [a Dart SDK
dependency](https://github.com/dart-lang/sdk/blob/main/pkg/macros/pubspec.yaml#L13);
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](https://github.com/flutter/flutter/issues/148004#issuecomment-2103099893));
and
4. The [flutter/flutter](https://github.com/flutter/flutter) repository
exactly pins the Dart SDK (via its exact pin on the
[flutter/engine](https://github.com/flutter/engine) repository
Therefore, the [flutter/flutter](https://github.com/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](https://github.com/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.
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