Make hasNext a getter instead of a method.

lib/coreimpl/hash_map_set.dart

Index: lib/coreimpl/hash_map_set.dart
diff --git a/lib/coreimpl/hash_map_set.dart b/lib/coreimpl/hash_map_set.dart
index cb723a553eb2b33d08072bed7edc232d70cc549e..6253aaec936d4d3605339186822edded3ffa0ec4 100644
--- a/lib/coreimpl/hash_map_set.dart
+++ b/lib/coreimpl/hash_map_set.dart
@@ -406,7 +406,7 @@ class HashSetIterator<E> implements Iterator<E> {
     _advance();
   }
 
-  bool hasNext() {
+  bool get hasNext {
     if (_nextValidIndex >= _entries.length) return false;
     if (_entries[_nextValidIndex] === HashMapImplementation._DELETED_KEY) {
       // This happens in case the set was modified in the meantime.
@@ -418,7 +418,7 @@ class HashSetIterator<E> implements Iterator<E> {
   }
 
   E next() {
-    if (!hasNext()) {
+    if (!hasNext) {
       throw const NoMoreElementsException();
     }
     E res = _entries[_nextValidIndex];