fix(DCCD): when watching variables, always fire on first digest

Author: mhevery

lib/change_detection/dirty_checking_change_detector.dart

@@ -467,7 +467,6 @@ class DirtyCheckingRecord<H> implements Record<H>, WatchRecord<H> {
       } else {
         _mode = _MODE_GETTER_;
         _getter = _fieldGetterFactory.getter(obj, field);
-        currentValue = _getter(obj);
       }
     }
   }

test/change_detection/dirty_checking_change_detector_spec.dart

@@ -743,6 +743,9 @@ void main() {
         Iterator changeIterator;
 
         detector..watch(user, 'isUnderAgeAsVariable', null);
+        changeIterator = detector.collectChanges();
+        expect(changeIterator.moveNext()).toEqual(true);
+
         changeIterator = detector.collectChanges();
         expect(changeIterator.moveNext()).toEqual(false);