Test Babel 7 plugins compatibility with Babel 8 core

babel.config.js

@@ -14,7 +14,7 @@ const pathUtils = require("path");
 const fs = require("fs");
 const { parseSync } = require("@babel/core");
 const packageJson = require("./package.json");
-const babel7_8compat = require("./babel-7-8-compatibility.json");
+const babel7_8compat = require("./test/babel-7-8-compat/data.json");
 
 function normalize(src) {
   return src.replace(/\//, pathUtils.sep);

test/babel-7-8-compat/babel-7-plugins-with-babel-8-core.sh

@@ -0,0 +1,42 @@
+#!/bin/bash
+
+# Usage: BABEL_8_VERSION=next ./test/babel-7-8-compat/babel-7-plugins-with-babel-8-core.sh
+# NOTE: This will change files in your monorepo, make sure to commit/stage
+# everything before running it.
+
+#==============================================================================#
+#                                  SETUP                                       #
+#==============================================================================#
+
+# Start in test/babel-7-8-compat even if run from root directory
+cd "$(dirname "$0")" || exit
+
+# Echo every command being executed
+set -x
+
+# Go to the root of the monorepo
+cd ../..
+
+#==============================================================================#
+#                                   TEST                                       #
+#==============================================================================#
+
+node -e '
+  if (!process.env.BABEL_8_VERSION) throw new Error("Please specify BABEL_8_VERSION=x.y.z env var");
+
+  const pluginsList = require("./test/babel-7-8-compat/data.json")["babel7plugins-babel8core"];
+
+  for (const plugin of pluginsList) {
+    const pkgJsonPath = path.join("./packages", plugin, "package.json");
+    const pkgJson = JSON.parse(fs.readFileSync(pkgJsonPath));
+    pkgJson.devDependencies["@babel/core"] = process.env.BABEL_8_VERSION;
+    fs.writeFileSync(pkgJsonPath, JSON.stringify(pkgJson, null, 2) + "\n");
+  }
+' || exit
+
+export YARN_ENABLE_IMMUTABLE_INSTALLS=false
+export SKIP_YARN_CORE_DEP_CHECK=true
+
+# Build and test
+make -j use-cjs
+yarn jest $(node -p 'require("./test/babel-7-8-compat/data.json")["babel7plugins-babel8core"].join(" ")' || exit)

test/babel-7-8-compat/data.json

@@ -0,0 +1,6 @@
+{
+  "babel7plugins-babel8core": [
+    "babel-plugin-external-helpers",
+    "babel-plugin-transform-json-strings"
+  ]
+}

yarn.config.cjs

@@ -268,6 +268,7 @@ function enforceBabelCoreNotInDeps({ Yarn }) {
       workspace.pkg.peerDependencies.has("@babel/core") &&
       !workspace.manifest.dependencies?.["@babel/core"]
     ) {
+      if (process.env.SKIP_YARN_CORE_DEP_CHECK) continue;
       workspace.set("devDependencies['@babel/core']", "workspace:^");
     }
     if (