Adding isNotEmpty property to collection and string.

tools/dom/src/AttributeMap.dart

Index: tools/dom/src/AttributeMap.dart
diff --git a/tools/dom/src/AttributeMap.dart b/tools/dom/src/AttributeMap.dart
index 6c96bc99a9a459f075fdde925165dc2708c1d583..b0cb0cc7fe6490a70f5a36d5650be6f63bf2d671 100644
--- a/tools/dom/src/AttributeMap.dart
+++ b/tools/dom/src/AttributeMap.dart
@@ -70,6 +70,11 @@ abstract class _AttributeMap implements Map<String, String> {
   }
 
   /**
+   * Returns true if there is at least one {key, value} pair in the map.
+   */
+  bool get isNotEmpty => !isEmpty;
+
+  /**
    * Checks to see if the node should be included in this map.
    */
   bool _matches(Node node);
@@ -216,6 +221,8 @@ class _DataAttributeMap implements Map<String, String> {
   // TODO: Use lazy iterator when it is available on Map.
   bool get isEmpty => length == 0;
 
+  bool get isNotEmpty => !isEmpty;
+
   // Helpers.
   String _attr(String key) => 'data-$key';
   bool _matches(String key) => key.startsWith('data-');