Skip write to 'name' property if not [[writable]]

Resolves: sinonjs#2203

lib/sinon/util/core/extend.js

@@ -73,8 +73,8 @@ function extendCommon(target, sources, doCopy) {
     return target;
 }
 
-/** Public: Extend target in place with all (own) properties from sources in-order. Thus, last source will
- *         override properties in previous sources.
+/** Public: Extend target in place with all (own) properties, except 'name' when [[writable]] is false,
+ *         from sources in-order. Thus, last source will override properties in previous sources.
  *
  * @arg {Object} target - The Object to extend
  * @arg {Object[]} sources - Objects to copy properties from.
@@ -85,7 +85,18 @@ module.exports = function extend(target /*, sources */) {
     var sources = slice(arguments, 1);
 
     return extendCommon(target, sources, function copyValue(dest, source, prop) {
-        dest[prop] = source[prop];
+        var destOwnPropertyDescriptor = Object.getOwnPropertyDescriptor(dest, prop);
+        var sourceOwnPropertyDescriptor = Object.getOwnPropertyDescriptor(source, prop);
+
+        // If the property is 'name' but is not [[writable]], skip it
+        if (prop !== "name" || destOwnPropertyDescriptor.writable) {
+            Object.defineProperty(dest, prop, {
+                configurable: sourceOwnPropertyDescriptor.configurable,
+                enumerable: sourceOwnPropertyDescriptor.enumerable,
+                writable: sourceOwnPropertyDescriptor.writable,
+                value: sourceOwnPropertyDescriptor.value
+            });
+        }
     });
 };