Make hasNext a getter instead of a method.

lib/coreimpl/queue.dart

Index: lib/coreimpl/queue.dart
diff --git a/lib/coreimpl/queue.dart b/lib/coreimpl/queue.dart
index 150752a7cf2f7dd81104037aae23210bb15f0545..6a7400280e646b2063facda56f758b26a1d94f93 100644
--- a/lib/coreimpl/queue.dart
+++ b/lib/coreimpl/queue.dart
@@ -249,12 +249,12 @@ class _DoubleLinkedQueueIterator<E> implements Iterator<E> {
     _currentEntry = _sentinel;
   }
 
-  bool hasNext() {
+  bool get hasNext {
     return _currentEntry._next !== _sentinel;
   }
 
   E next() {
-    if (!hasNext()) {
+    if (!hasNext) {
       throw const NoMoreElementsException();
     }
     _currentEntry = _currentEntry._next;