Tag closures with their types

test/codegen/expect/covariance.js

 var covariance = dart.defineLibrary(covariance, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   let _t = Symbol('_t');
   let Foo$ = dart.generic(function(T) {
     class Foo extends core.Object {
       Foo() {
         this[_t] = null;
       }
       add(t) {
         dart.as(t, T);
         this[_t] = t;
       }
       forEach(fn) {
         dart.as(fn, dart.functionType(dart.void, [T]));
         fn(this[_t]);
       }
     }
+    dart.setSignature(Foo, {
+      methods: () => ({
+        add: dart.functionType(dart.dynamic, [T]),
+        forEach: dart.functionType(dart.dynamic, [dart.functionType(dart.void, [T])])
+      })
+    });
     return Foo;
   });
   let Foo = Foo$();
   class Bar extends Foo$(core.int) {
     Bar() {
       super.Foo();
     }
     add(x) {
       core.print(`Bar.add got ${x}`);
       super.add(x);
     }
   }
-  // Function main: () → dynamic
+  dart.setSignature(Bar, {
+    methods: () => ({add: dart.functionType(dart.dynamic, [core.int])})
+  });
   function main() {
     let foo = new Bar();
     foo.add('hi');
   }
+  dart.fn(main);
   // Exports:
   exports.Foo$ = Foo$;
   exports.Foo = Foo;
   exports.Bar = Bar;
   exports.main = main;
 })(covariance, core);