Make isEmpty a getter.

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 897fcc66100cc9e6ec5818269381b146a46cd72c..fe4a671557407e0ba049aea891f5f2522b34c4fa 100644
--- a/lib/coreimpl/hash_map_set.dart
+++ b/lib/coreimpl/hash_map_set.dart
@@ -219,7 +219,7 @@ class HashMapImplementation<K, V> implements HashMap<K, V> {
     return null;
   }
 
-  bool isEmpty() {
+  bool get isEmpty {
     return _numberOfEntries == 0;
   }
 
@@ -375,8 +375,8 @@ class HashSetImplementation<E > implements HashSet<E> {
     return keys.some(f);
   }
 
-  bool isEmpty() {
-    return _backingMap.isEmpty();
+  bool get isEmpty {
+    return _backingMap.isEmpty;
   }
 
   int get length {