Add constructors to signatures

test/codegen/expect/sunflower/dom.js

 var dom = dart.defineLibrary(dom, window);
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   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, [], {am: core.String}]})
+  });
   class Overload extends core.Object {
     Overload() {
     }
   }
+  dart.setSignature(Overload, {
+    constructors: () => ({Overload: [Overload, []]})
+  });
   let overload = dart.const(new Overload());
   let EventListener = dart.typedef('EventListener', () => dart.functionType(dart.void, [Event]));
   class Event extends core.Object {}
   let InputElement = HTMLInputElement;
   let CanvasElement = HTMLCanvasElement;
   class RenderingContext extends core.Object {}
   class CanvasDrawingStyles extends core.Object {
     CanvasDrawingStyles() {
       this.lineWidth = null;
       this.lineCap = null;
@@ skipping 11 matching lines @@
   exports.Overload = Overload;
   exports.overload = overload;
   exports.EventListener = EventListener;
   exports.Event = Event;
   exports.InputElement = InputElement;
   exports.CanvasElement = CanvasElement;
   exports.RenderingContext = RenderingContext;
   exports.CanvasDrawingStyles = CanvasDrawingStyles;
   exports.CanvasPathMethods = CanvasPathMethods;
 })(dom, core);