Add constructors to signatures

test/codegen/expect/names.js

 var names = dart.defineLibrary(names, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   exports.exports = 42;
   let _foo$ = Symbol('_foo');
   class Foo extends core.Object {
     [_foo$]() {
       return 123;
     }
   }
   dart.setSignature(Foo, {
     methods: () => ({[_foo$]: [core.Object, []]})
   });
   function _foo() {
     return 456;
   }
   dart.fn(_foo);
   class Frame extends core.Object {
     caller(arguments$) {
       this.arguments = arguments$;
     }
     static callee() {
       return null;
     }
   }
   dart.defineNamedConstructor(Frame, 'caller');
   dart.setSignature(Frame, {
+    constructors: () => ({caller: [Frame, [core.List]]}),
     statics: () => ({callee: [core.Object, []]}),
     names: ['callee']
   });
   class Frame2 extends core.Object {}
   dart.defineLazyProperties(Frame2, {
     get caller() {
       return 100;
     },
     set caller(_) {},
     get arguments() {
@@ skipping 10 matching lines @@
     core.print(eval$);
     core.print(dart.notNull(Frame2.caller) + dart.notNull(Frame2.arguments));
   }
   dart.fn(main);
   // Exports:
   exports.Foo = Foo;
   exports.Frame = Frame;
   exports.Frame2 = Frame2;
   exports.main = main;
 })(names, core);