Prevent monkey patching

This doesn't affect globals (describe, it, expect, etc). Those belong to
the user and Jasmine doesn't depend on them.

src/core/Timer.js

@@ -39,7 +39,11 @@ getJasmineRequireObj().Timer = function() {
     this.elapsed = function() {
       return now() - startTime;
     };
+
+    Object.freeze(this);
   }
 
+  Object.freeze(Timer);
+  Object.freeze(Timer.prototype);
   return Timer;
 };