Update allowed_experiments.json to enable macros for the json package.

Without this there is no way to enable the experiment for a dependency, so nobody can try out package:json.

Bug: https://github.com/dart-lang/sdk/issues/55688
Change-Id: Id2f10b10dde0eb6d68d0bf76030ff557087ee83b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/366041
Reviewed-by: Leaf Petersen <leafp@google.com>
Auto-Submit: Jake Macdonald <jakemac@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Leaf Petersen <leafp@google.com>
diff --git a/pkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart b/pkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart
index a4cbe06..9d818ff 100644
--- a/pkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart
+++ b/pkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart
@@ -819,7 +819,11 @@
     const AllowedExperimentalFlags(
         sdkDefaultExperiments: {},
         sdkLibraryExperiments: {},
-        packageExperiments: {});
+        packageExperiments: {
+      "json": {
+        ExperimentalFlag.macros,
+      },
+    });
 const Map<shared.ExperimentalFlag, ExperimentalFlag> sharedExperimentalFlags = {
   shared.ExperimentalFlag.classModifiers: ExperimentalFlag.classModifiers,
   shared.ExperimentalFlag.constFunctions: ExperimentalFlag.constFunctions,
diff --git a/sdk/lib/_internal/allowed_experiments.json b/sdk/lib/_internal/allowed_experiments.json
index b51be26..b7049f9 100644
--- a/sdk/lib/_internal/allowed_experiments.json
+++ b/sdk/lib/_internal/allowed_experiments.json
@@ -1,7 +1,10 @@
 {
   "version": 1,
   "experimentSets": {
-    "sdkExperiments": []
+    "sdkExperiments": [],
+    "macros": [
+      "macros"
+    ]
   },
   "sdk": {
     "default": {
@@ -13,5 +16,9 @@
       }
     }
   },
-  "packages": {}
+  "packages": {
+    "json": {
+      "experimentSet": "macros"
+    }
+  }
 }