Stop pinning deps in working/macros/example/pubspec.yaml. (#3595)

Pinning the deps was creating a version skew problem in continuous
integration (which is configured in `.github/workflows/dart.yml`). The
continuous integration configuration uses `sdk: main` (meaning that
the code will be analyzed using the latest bleeding edge build of
Dart). But the macros in `working/macros/example` import macro support
functionality from `package:_fe_analyzer_shared/src`, so they use
whatever version of that package that is configured in the pubspec
(which, prior to this change, was pinned to version `3.4.0-56.0.dev`).

The reason this creates a version skew problem is that the macro
support functionality in `package:_fe_analyzer_shared/src` includes
deserialization logic that decodes data structures created during
analysis and compilation. Which means that any time the serialization
format changes, that could potentially break the continuous
integration bot.

Eventually we plan to move the macro support functionality into a
`dart:` library that will ship as part of the SDK, so there will be no
more version skew problem. Until then, an easy way to work around the
version skew is simply to pin the pubspec to the `main` branch rather
than a specific release, so that it matches what's configured in
continuous integration.

A side effect of this change is that if the API for writing macros
gets changed in a way that breaks the examples in
`working/macros/example`, that will cause the continuous integration
bot to immediately start failing. (Previously it wouldn't cause a
failure because the API was pinned to `3.4.0-56.0.dev`). Considering
that the API for macros is under active development, this is probably
a good thing; it will mean that if we make inadvertent API changes
that break the examples, we'll be more likely to find out about it.

Author: stereotype441

working/macros/example/pubspec.yaml

@@ -17,84 +17,84 @@ dependency_overrides:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/_fe_analyzer_shared
-      ref: 3.4.0-56.0.dev
+      ref: main
   _js_interop_checks:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/_js_interop_checks
-      ref: 3.4.0-56.0.dev
+      ref: main
   analyzer:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/analyzer
-      ref: 3.4.0-56.0.dev
+      ref: main
   build_integration:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/build_integration
-      ref: 3.4.0-56.0.dev
+      ref: main
   compiler:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/compiler
-      ref: 3.4.0-56.0.dev
+      ref: main
   dart2js_info:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/dart2js_info
-      ref: 3.4.0-56.0.dev
+      ref: main
   dart2wasm:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/dart2wasm
-      ref: 3.4.0-56.0.dev
+      ref: main
   dev_compiler:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/dev_compiler
-      ref: 3.4.0-56.0.dev
+      ref: main
   front_end:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/front_end
-      ref: 3.4.0-56.0.dev
+      ref: main
   frontend_server:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/frontend_server
-      ref: 3.4.0-56.0.dev
+      ref: main
   js_ast:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/js_ast
-      ref: 3.4.0-56.0.dev
+      ref: main
   js_runtime:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/js_runtime
-      ref: 3.4.0-56.0.dev
+      ref: main
   js_shared:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/js_shared
-      ref: 3.4.0-56.0.dev
+      ref: main
   kernel:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/kernel
-      ref: 3.4.0-56.0.dev
+      ref: main
   mmap:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/mmap
-      ref: 3.4.0-56.0.dev
+      ref: main
   vm:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/vm
-      ref: 3.4.0-56.0.dev
+      ref: main
   wasm_builder:
     git:
       url: https://github.com/dart-lang/sdk.git
       path: pkg/wasm_builder
-      ref: 3.4.0-56.0.dev
+      ref: main