fix #27259, implement covariance checking for strong mode and DDC

pkg/dev_compiler/test/codegen_expected/closure.js

 export const closure = Object.create(null);
 import { core, js, dart, dartx } from 'dart_sdk';
 let dynamic__Toint = () => (dynamic__Toint = dart.constFn(dart.fnTypeFuzzy(core.int, [dart.dynamic], [dart.dynamic])))();
 let dynamic__Todynamic = () => (dynamic__Todynamic = dart.constFn(dart.fnTypeFuzzy(dart.dynamic, [dart.dynamic], {y: core.String, z: dart.dynamic})))();
 let StringToList = () => (StringToList = dart.constFn(dart.fnTypeFuzzy(core.List, [core.String])))();
 let ListOfint = () => (ListOfint = dart.constFn(core.List$(core.int)))();
 let dynamicTodynamic = () => (dynamicTodynamic = dart.constFn(dart.fnTypeFuzzy(dart.dynamic, [dart.dynamic])))();
 let FnToListOfint = () => (FnToListOfint = dart.constFn(dart.fnTypeFuzzy(ListOfint(), [dynamicTodynamic()])))();
 let MapOfMap$Map = () => (MapOfMap$Map = dart.constFn(core.Map$(core.Map, core.Map)))();
 let Foo = () => (Foo = dart.constFn(closure.Foo$()))();
@@ skipping 33 matching lines @@
     get s() {
       return this[s];
     }
     set s(value) {
       this[s] = value;
     }
     get v() {
       return this[v$];
     }
     set v(value) {
-      this[v$] = value;
+      this[v$] = T._check(value);
     }
     static build() {
       return new (FooOfT()).new(1, null);
     }
     untyped_method(a, b) {}
     pass(t: T) {
       T._check(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) {
@@ skipping 87 matching lines @@
     return "abc";
   },
   get some_top_level_var() {
     return "abc";
   },
   set some_top_level_var(_) {}
 });
 dart.trackLibraries("closure", {
   "closure.dart": closure
 }, null);