Adding isNotEmpty property to collection and string.

pkg/serialization/lib/src/serialization_helpers.dart

Index: pkg/serialization/lib/src/serialization_helpers.dart
diff --git a/pkg/serialization/lib/src/serialization_helpers.dart b/pkg/serialization/lib/src/serialization_helpers.dart
index 9568c24dee7db223fe32fb46df89a1c06c8d0da3..1f86a9777a60947afcec8382ed5095fbb7485936 100644
--- a/pkg/serialization/lib/src/serialization_helpers.dart
+++ b/pkg/serialization/lib/src/serialization_helpers.dart
@@ -248,6 +248,7 @@ class IdentityMap<K, V> implements Map<K, V> {
     values.clear();
   }
   bool get isEmpty => keys.isEmpty;
+  bool get isNotEmpty => !isEmpty;
 
   // Note that this is doing an equality comparison.
   bool containsValue(x) => values.contains(x);