Abort spying when the target cannot be spied on

src/core/SpyRegistry.js

@@ -22,6 +22,11 @@ getJasmineRequireObj().SpyRegistry = function(j$) {
         throw new Error(methodName + ' has already been spied upon');
       }
 
+      var descriptor = Object.getOwnPropertyDescriptor(obj, methodName);
+      if (!(descriptor.writable || descriptor.set)) {
+        throw new Error(methodName + ' is not declared writable or has no setter');
+      }
+
       var spy = j$.createSpy(methodName, obj[methodName]);
 
       currentSpies().push({