Switch ddc to use @JS instead of @JSName

test/codegen/expect/dom/dom.js

 dart_library.library('dom/dom', window, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
-  class JsName extends core.Object {
-    JsName(opts) {
-      let name = opts && 'name' in opts ? opts.name : null;
-      this.name = name;
-    }
-  }
-  dart.setSignature(JsName, {
-    constructors: () => ({JsName: [JsName, [], {name: core.String}]})
-  });
   class Overload extends core.Object {
     Overload() {
     }
   }
   dart.setSignature(Overload, {
     constructors: () => ({Overload: [Overload, []]})
   });
   let overload = dart.const(new Overload());
   class CustomEvent extends core.Object {}
   dart.setSignature(CustomEvent, {
@@ skipping 16 matching lines @@
       this.lineJoin = null;
       this.miterLimit = null;
       this.lineDashOffset = null;
       this.font = null;
       this.textAlign = null;
       this.textBaseline = null;
     }
   }
   class CanvasPathMethods extends core.Object {}
   // Exports:
-  exports.JsName = JsName;
   exports.Overload = Overload;
   exports.overload = overload;
   exports.CustomEvent = CustomEvent;
   exports.HTMLCollection = HTMLCollection;
   exports.EventListener = EventListener;
   exports.InputElement = InputElement;
   exports.CanvasElement = CanvasElement;
   exports.DivElement = DivElement;
   exports.ScriptElement = ScriptElement;
   exports.RenderingContext = RenderingContext;
   exports.CanvasDrawingStyles = CanvasDrawingStyles;
   exports.CanvasPathMethods = CanvasPathMethods;
 });