implement mixins in subtype checks, more codegen fixes

test/codegen/expect/sunflower/dom.js

 var dom;
 (function(exports) {
   'use strict';
   class JsName extends core.Object {
     JsName(opts) {
       let name = opts && 'name' in opts ? opts.name : null;
       this.name = name;
     }
   }
   class Overload extends core.Object {
     Overload() {
     }
   }
   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;
       this.lineCap = null;
       this.lineJoin = null;
       this.miterLimit = null;
       this.lineDashOffset = null;
       this.font = null;
       this.textAlign = null;
       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;
   exports.CanvasPathMethods = CanvasPathMethods;
 })(dom || (dom = window));