Throw error for multiple exports default (#3518)

Fixes https://phabricator.babeljs.io/T7242

packages/babel-plugin-transform-es2015-modules-commonjs/src/index.js

@@ -170,6 +170,8 @@ export default function () {
 
           let requires = Object.create(null);
 
+          let exportDefaultFound = false;
+
           function addRequire(source, blockHoist) {
             let cached = requires[source];
             if (cached) return cached;
@@ -238,6 +240,10 @@ export default function () {
 
               path.remove();
             } else if (path.isExportDefaultDeclaration()) {
+              if (exportDefaultFound) {
+                throw path.buildCodeFrameError("Only one default export allowed per module.");
+              }
+
               let declaration = path.get("declaration");
               if (declaration.isFunctionDeclaration()) {
                 let id = declaration.node.id;
@@ -270,6 +276,7 @@ export default function () {
                 // replaceWith above. See T7166 for more info.
                 path.parentPath.requeue(path.get("expression.left"));
               }
+              exportDefaultFound = true;
             } else if (path.isExportNamedDeclaration()) {
               let declaration = path.get("declaration");
               if (declaration.node) {