Prep to release package:macros

- update SDK dependency to ensure it supports SDK vendored packages
- sync pubspec/changelog versions

Change-Id: Ib79244bec43fc94bf12340241f6eab4001e4b21c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/359920
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>
Auto-Submit: Jake Macdonald <jakemac@google.com>

pkg/_macros/pubspec.yaml

@@ -8,7 +8,7 @@ publish_to: none
 repository: https://github.com/dart-lang/sdk/tree/main/pkg/_macros
 
 environment:
-  sdk: ^3.3.0
+  sdk: ^3.4.0-256.0.dev
 
 # Note that as an SDK vendored package, pub package dependencies are only
 # allowed in the dev_dependencies section.

pkg/macros/CHANGELOG.md

@@ -1,3 +1,3 @@
-## 0.1.0
+## 0.1.0-main.0
 
 Initial release, highly experimental at this time.

pkg/macros/pubspec.yaml

@@ -6,7 +6,7 @@ description: >-
 repository: https://github.com/dart-lang/sdk/tree/main/pkg/macros
 
 environment:
-  sdk: ^3.3.0
+  sdk: ^3.4.0-256.0.dev
 
 dependencies:
   _macros: