Extract ES Module detection and improve error

lib/sinon/stub.js

@@ -6,6 +6,7 @@ var spy = require("./spy");
 var extend = require("./util/core/extend");
 var functionToString = require("./util/core/function-to-string");
 var getPropertyDescriptor = require("./util/core/get-property-descriptor");
+var isEsModule = require("./util/core/is-es-module");
 var wrapMethod = require("./util/core/wrap-method");
 var stubEntireObject = require("./stub-entire-object");
 var throwOnFalsyObject = require("./throw-on-falsy-object");
@@ -18,12 +19,8 @@ function stub(object, property) {
         throw new TypeError("stub(obj, 'meth', fn) has been removed, see documentation");
     }
 
-    if (
-        object &&
-        typeof Symbol !== "undefined" &&
-        object[Symbol.toStringTag] === "Module"
-    ) {
-        throw new TypeError("No support for stubbing ES Modules");
+    if (isEsModule(object)) {
+        throw new TypeError("ES Modules cannot be stubbed");
     }
 
     throwOnFalsyObject.apply(null, arguments);

lib/sinon/util/core/is-es-module.js

@@ -0,0 +1,16 @@
+"use strict";
+
+/**
+ * Verify if an object is a ECMAScript Module
+ *
+ * As the exports from a module is immutable we cannot alter the exports
+ * using spies or stubs. Let the consumer know this to avoid bug reports
+ * on weird error messages.
+ */
+module.exports = function (object) {
+    return (
+        object &&
+        typeof Symbol !== "undefined" &&
+        object[Symbol.toStringTag] === "Module"
+    );
+};