Added a positional Component.fromArgs() constructor

To define a Component subclass with mixins, a constructor without optional parameters is needed.

Added Component.fromArgs() which just delegates to the main constructor.

R=rafaelw@chromium.org

Review URL: https://codereview.chromium.org/1017353002

framework/fn.dart

@@ -653,6 +653,9 @@ abstract class Component extends Node {
         _order = _currentOrder + 1,
         super(key:key);
 
+  Component.fromArgs(Object key, bool stateful)
+      : this(key: key, stateful: stateful);
+
   void didMount() {}
   void didUnmount() {}