add custom_element package

pkg/observe/lib/src/observable_map.dart

Index: pkg/observe/lib/src/observable_map.dart
diff --git a/pkg/observe/lib/src/observable_map.dart b/pkg/observe/lib/src/observable_map.dart
index 2cd64d57d1b52749b4cac6a6b5b47e524cea709b..621c86fa5afc413a2a4dea5cb009909153b93bb8 100644
--- a/pkg/observe/lib/src/observable_map.dart
+++ b/pkg/observe/lib/src/observable_map.dart
@@ -67,9 +67,7 @@ class ObservableMap<K, V> extends ChangeNotifierBase implements Map<K, V> {
    * you should use [toObservable].
    */
   factory ObservableMap.from(Map<K, V> other) {
-    var result = new ObservableMap<K, V>._createFromType(other);
-    other.forEach((key, value) { result[key] = value; });
-    return result;
+    return new ObservableMap<K, V>._createFromType(other)..addAll(other);
   }
 
   factory ObservableMap._createFromType(Map<K, V> other) {
@@ -114,6 +112,10 @@ class ObservableMap<K, V> extends ChangeNotifierBase implements Map<K, V> {
     }
   }
 
+  void addAll(Map<K, V> other) {
+    other.forEach((K key, V value) { this[key] = value; });
+  }
+
   V putIfAbsent(K key, V ifAbsent()) {
     int len = _map.length;
     V result = _map.putIfAbsent(key, ifAbsent);