Throw if the argument to jasmine.clock().mockDate() is not a Date

src/core/MockDate.js

@@ -17,10 +17,9 @@ getJasmineRequireObj().MockDate = function(j$) {
         currentTime = mockDate.getTime();
       } else {
         if (!j$.util.isUndefined(mockDate)) {
-          j$.getEnv().deprecated(
+          throw new Error(
             'The argument to jasmine.clock().mockDate(), if specified, ' +
-              'should be a Date instance. Passing anything other than a Date ' +
-              'will be treated as an error in a future release.'
+              'should be a Date instance.'
           );
         }