Split export extensions into 2 different plugins, update stage presets (#6080)

packages/babel-plugin-transform-export-default/src/index.js

@@ -0,0 +1,46 @@
+import syntaxExportExtensions from "babel-plugin-syntax-export-extensions";
+
+export default function({ types: t }) {
+  function build(node, nodes, scope) {
+    const hasNoExportDefault = node.specifiers.every(
+      specifier => !t.isExportDefaultSpecifier(specifier),
+    );
+    if (hasNoExportDefault) return;
+
+    const specifier = node.specifiers.shift();
+    const uid = scope.generateUidIdentifier(specifier.exported.name);
+
+    let newSpecifier;
+    if (t.isExportNamespaceSpecifier(specifier)) {
+      newSpecifier = t.importNamespaceSpecifier(uid);
+    } else {
+      newSpecifier = t.importDefaultSpecifier(uid);
+    }
+    nodes.push(t.importDeclaration([newSpecifier], node.source));
+    nodes.push(
+      t.exportNamedDeclaration(null, [
+        t.exportSpecifier(uid, specifier.exported),
+      ]),
+    );
+
+    build(node, nodes, scope);
+  }
+
+  return {
+    inherits: syntaxExportExtensions,
+
+    visitor: {
+      ExportNamedDeclaration(path) {
+        const { node, scope } = path;
+        const nodes = [];
+        build(node, nodes, scope);
+        if (!nodes.length) return;
+
+        if (node.specifiers.length >= 1) {
+          nodes.push(node);
+        }
+        path.replaceWithMultiple(nodes);
+      },
+    },
+  };
+}