Eliminate unnecessary covariance checks.

test/codegen/expect/closure.js

 export const closure = Object.create(null);
 import { core, js, dart, dartx } from 'dart_sdk';
 closure.generic_function = function(T) {
   return (items: core.List<T>, seed: T): core.List<T> => {
     let strings = items[dartx.map](core.String)(dart.fn((i: T): string => `${i}`, core.String, [T]))[dartx.toList]();
     return items;
   };
 };
 dart.fn(closure.generic_function, T => [core.List$(T), [core.List$(T), T]]);
 closure.Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
@@ skipping 20 matching lines @@
       dart.as(t, T);
       return t;
     }
     typed_method(foo: closure.Foo<any>, list: core.List<any>, i: number, n: number, d: number, b: boolean, s: string, a: any[], o: Object, f: Function) {
       return '';
     }
     optional_params(a, b = null, c: number = null) {}
     static named_params(a, {b = null, c = null}: {b?: any, c?: number} = {}) {}
     nullary_method() {}
     function_params(f: (x: any, y?: any) => number, g: (x: any, opts?: {y?: string, z?: any}) => any, cb: closure.Callback) {
-      dart.as(f, dart.functionType(core.int, [dart.dynamic], [dart.dynamic]));
-      dart.as(g, dart.functionType(dart.dynamic, [dart.dynamic], {y: core.String, z: dart.dynamic}));
       cb({i: this.i});
     }
-    run(a: core.List<any>, b: string, c: (d: string) => core.List<any>, e: (f: (g: any) => any) => core.List<number>, {h = null}: {h?: core.Map<core.Map<any, any>, core.Map<any, any>>} = {}) {
-      dart.as(c, dart.functionType(core.List, [core.String]));
-      dart.as(e, dart.functionType(core.List$(core.int), [dart.functionType(dart.dynamic, [dart.dynamic])]));
-    }
+    run(a: core.List<any>, b: string, c: (d: string) => core.List<any>, e: (f: (g: any) => any) => core.List<number>, {h = null}: {h?: core.Map<core.Map<any, any>, core.Map<any, any>>} = {}) {}
     get prop() {
       return null;
     }
     set prop(value: string) {}
     static get staticProp() {
       return null;
     }
     static set staticProp(value: string) {}
   }
   dart.setSignature(Foo, {
@@ skipping 33 matching lines @@
 });
 closure.main = function(args): void {
 };
 dart.fn(closure.main, dart.void, [dart.dynamic]);
 /** @final {string} */
 closure.some_top_level_constant = "abc";
 /** @final {string} */
 closure.some_top_level_final = "abc";
 /** @type {string} */
 closure.some_top_level_var = "abc";