implement mixins in subtype checks, more codegen fixes

test/codegen/expect/sunflower/dom.js

Index: test/codegen/expect/sunflower/dom.js
diff --git a/test/codegen/expect/sunflower/dom.js b/test/codegen/expect/sunflower/dom.js
index 62d83cdbe91307e23f5b0fc36916cbd4c605fbcd..82014229d5c35301a7d9e0e8a0a1d8dffeff250a 100644
--- a/test/codegen/expect/sunflower/dom.js
+++ b/test/codegen/expect/sunflower/dom.js
@@ -12,10 +12,9 @@ var dom;
     }
   }
   let overload = new Overload();
-  class Event extends core.Object {
-  }
-  class RenderingContext extends core.Object {
-  }
+  class EventListener extends core.Function {}
+  class Event extends core.Object {}
+  class RenderingContext extends core.Object {}
   class CanvasDrawingStyles extends core.Object {
     CanvasDrawingStyles() {
       this.lineWidth = null;
@@ -28,12 +27,12 @@ var dom;
       this.textBaseline = null;
     }
   }
-  class CanvasPathMethods extends core.Object {
-  }
+  class CanvasPathMethods extends core.Object {}
   // Exports:
   exports.JsName = JsName;
   exports.Overload = Overload;
   exports.overload = overload;
+  exports.EventListener = EventListener;
   exports.Event = Event;
   exports.RenderingContext = RenderingContext;
   exports.CanvasDrawingStyles = CanvasDrawingStyles;