Add support for BABEL_8_BREAKING flag (stripped on publish)

Gulpfile.js

@@ -110,7 +110,7 @@ if (process.env.CIRCLE_PR_NUMBER) {
 
 const babelVersion =
   require("./packages/babel-core/package.json").version + versionSuffix;
-function buildRollup(packages) {
+function buildRollup(packages, targetBrowsers) {
   const sourcemap = process.env.NODE_ENV === "production";
   return Promise.all(
     packages.map(async ({ src, format, dest, name, filename }) => {
@@ -166,11 +166,12 @@ function buildRollup(packages) {
             ],
           }),
           rollupJson(),
-          rollupNodePolyfills({
-            sourceMap: sourcemap,
-            include: "**/*.{js,ts}",
-          }),
-        ],
+          targetBrowsers &&
+            rollupNodePolyfills({
+              sourceMap: sourcemap,
+              include: "**/*.{js,ts}",
+            }),
+        ].filter(Boolean),
       });
 
       const outputFile = path.join(src, dest, filename || "index.js");
@@ -235,7 +236,7 @@ const standaloneBundle = [
 ];
 
 gulp.task("build-rollup", () => buildRollup(libBundles));
-gulp.task("build-babel-standalone", () => buildRollup(standaloneBundle));
+gulp.task("build-babel-standalone", () => buildRollup(standaloneBundle, true));
 
 gulp.task("build-babel", () => buildBabel(/* exclude */ libBundles));
 gulp.task("build", gulp.parallel("build-rollup", "build-babel"));

Makefile

@@ -206,7 +206,7 @@ clone-license:
 	./scripts/clone-license.sh
 
 prepublish-build: clean-lib clean-runtime-helpers
-	NODE_ENV=production BABEL_ENV=production $(MAKE) build-bundle
+	NODE_ENV=production BABEL_ENV=production STRIP_BABEL_8_FLAG=true $(MAKE) build-bundle
 	$(MAKE) prepublish-build-standalone clone-license
 
 prepublish:

babel.config.js

@@ -77,6 +77,11 @@ module.exports = function (api) {
       break;
   }
 
+  if (process.env.STRIP_BABEL_8_FLAG && bool(process.env.BABEL_8_BREAKING)) {
+    // Never apply polyfills when compiling for Babel 8
+    polyfillRequireResolve = false;
+  }
+
   if (includeRegeneratorRuntime) {
     const babelRuntimePkgPath = require.resolve("@babel/runtime/package.json");
 
@@ -119,6 +124,11 @@ module.exports = function (api) {
 
       convertESM ? "@babel/proposal-export-namespace-from" : null,
       convertESM ? "@babel/transform-modules-commonjs" : null,
+
+      process.env.STRIP_BABEL_8_FLAG && [
+        pluginToggleBabel8Breaking,
+        { breaking: bool(process.env.BABEL_8_BREAKING) },
+      ],
       polyfillRequireResolve && pluginPolyfillRequireResolve,
     ].filter(Boolean),
     overrides: [
@@ -165,6 +175,11 @@ module.exports = function (api) {
   return config;
 };
 
+// env vars from the cli are always strings, so !!ENV_VAR returns true for "false"
+function bool(value) {
+  return value && value === "false" && value === "0";
+}
+
 // TODO(Babel 8) This polyfill is only needed for Node.js 6 and 8
 function pluginPolyfillRequireResolve({ template, types: t }) {
   return {
@@ -204,3 +219,32 @@ function pluginPolyfillRequireResolve({ template, types: t }) {
     },
   };
 }
+
+function pluginToggleBabel8Breaking({ types: t }, { breaking }) {
+  return {
+    visitor: {
+      "IfStatement|ConditionalExpression"(path) {
+        let test = path.get("test");
+        let keepConsequent = breaking;
+
+        if (test.isUnaryExpression({ operator: "!" })) {
+          test = test.get("argument");
+          keepConsequent = !keepConsequent;
+        }
+
+        if (!test.matchesPattern("process.env.BABEL_8_BREAKING")) return;
+
+        path.replaceWith(
+          keepConsequent
+            ? path.node.consequent
+            : path.node.alternate || t.emptyStatement()
+        );
+      },
+      MemberExpression(path) {
+        if (path.matchesPattern("process.env.BABEL_8_BREAKING")) {
+          throw path.buildCodeFrameError("This check could not be stripped.");
+        }
+      },
+    },
+  };
+}

packages/babel-helper-fixtures/src/index.js

@@ -122,7 +122,11 @@ function pushTask(taskName, taskDir, suite, suiteName) {
   const test = {
     optionsDir: taskOptsLoc ? path.dirname(taskOptsLoc) : null,
     title: humanize(taskName, true),
-    disabled: taskName[0] === ".",
+    disabled:
+      taskName[0] === "." ||
+      (process.env.BABEL_8_BREAKING
+        ? taskOpts.BABEL_8_BREAKING === false
+        : taskOpts.BABEL_8_BREAKING === true),
     options: taskOpts,
     validateLogs: taskOpts.validateLogs,
     ignoreOutput: taskOpts.ignoreOutput,