Change NotImplementedException to UnimplementedError.

lib/html/templates/html/impl/impl_Element.darttemplate

Index: lib/html/templates/html/impl/impl_Element.darttemplate
diff --git a/lib/html/templates/html/impl/impl_Element.darttemplate b/lib/html/templates/html/impl/impl_Element.darttemplate
index e8b9ed593a3ab2988f70ba18daa52d159ddf2e2e..bc0f0a79ec5561ed4d7fae572fd4d4a1d455b1a0 100644
--- a/lib/html/templates/html/impl/impl_Element.darttemplate
+++ b/lib/html/templates/html/impl/impl_Element.darttemplate
@@ -106,15 +106,15 @@ class _ChildrenElementList implements List {
   }
 
   void setRange(int start, int rangeLength, List from, [int startFrom = 0]) {
-    throw const NotImplementedException();
+    throw new UnimplementedError();
   }
 
   void removeRange(int start, int rangeLength) {
-    throw const NotImplementedException();
+    throw new UnimplementedError();
   }
 
   void insertRange(int start, int rangeLength, [initialValue = null]) {
-    throw const NotImplementedException();
+    throw new UnimplementedError();
   }
 
   List getRange(int start, int rangeLength) =>