enable the allowUndeclaredExports Babel option (#6660)

src/language-js/parser-babylon.js

@@ -16,6 +16,7 @@ function babelOptions(extraOptions, extraPlugins) {
       allowImportExportEverywhere: true,
       allowReturnOutsideFunction: true,
       allowSuperOutsideMethod: true,
+      allowUndeclaredExports: true,
       plugins: [
         "jsx",
         "doExpressions",

tests/exports/__snapshots__/jsfmt.spec.js.snap

@@ -14,6 +14,8 @@ export * as ns from "mod";
 
 export * as foo,{bar} from "./baz";
 
+export { undefinedExport };
+
 =====================================output=====================================
 export {
   value1,
@@ -29,5 +31,7 @@ export * as ns from "mod";
 
 export * as foo, { bar } from "./baz";
 
+export { undefinedExport };
+
 ================================================================================
 `;

tests/exports/test.js

@@ -5,3 +5,5 @@ export a,{b} from "./baz";
 export * as ns from "mod";
 
 export * as foo,{bar} from "./baz";
+
+export { undefinedExport };