[@babel/parser] Add "allowUndeclaredExports" option (#9864)

Ref: https://github.com/babel/notes/blob/master/2019/04/09.md#export-changes-in-scope-pr-break-meteor-issue

packages/babel-parser/src/options.js

@@ -15,6 +15,7 @@ export type Options = {
   allowReturnOutsideFunction: boolean,
   allowImportExportEverywhere: boolean,
   allowSuperOutsideMethod: boolean,
+  allowUndeclaredExports: boolean,
   plugins: PluginList,
   strictMode: ?boolean,
   ranges: boolean,
@@ -41,6 +42,8 @@ export const defaultOptions: Options = {
   allowImportExportEverywhere: false,
   // TODO
   allowSuperOutsideMethod: false,
+  // When enabled, export statements can reference undeclared variables.
+  allowUndeclaredExports: false,
   // An array of plugins to enable
   plugins: [],
   // TODO

packages/babel-parser/src/parser/statement.js

@@ -45,7 +45,11 @@ export default class StatementParser extends ExpressionParser {
 
     this.parseBlockBody(program, true, true, tt.eof);
 
-    if (this.inModule && this.scope.undefinedExports.size > 0) {
+    if (
+      this.inModule &&
+      !this.options.allowUndeclaredExports &&
+      this.scope.undefinedExports.size > 0
+    ) {
       for (const [name] of Array.from(this.scope.undefinedExports)) {
         const pos = this.scope.undefinedExports.get(name);
         // $FlowIssue