Eliminate unnecessary covariance checks.

test/codegen/expect/covariance.js

 dart_library.library('covariance', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const covariance = Object.create(null);
   const _t = Symbol('_t');
   covariance.Foo$ = dart.generic(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.dynamic, [T]],
         forEach: [dart.dynamic, [dart.functionType(dart.void, [T])]]
       })
     });
     return Foo;
@@ skipping 12 matching lines @@
     methods: () => ({add: [dart.dynamic, [core.int]]})
   });
   covariance.main = function() {
     let foo = new covariance.Bar();
     foo.add('hi');
   };
   dart.fn(covariance.main);
   // Exports:
   exports.covariance = covariance;
 });