fix #30423, covariant parameter tearoff type should be Object

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$()))();
@@ skipping 84 matching lines @@
     fields: () => ({
       i: dart.finalFieldType(core.int),
       b: dart.fieldType(core.bool),
       s: dart.fieldType(core.String),
       v: dart.fieldType(T)
     }),
     getters: () => ({prop: dart.fnType(core.String, [])}),
     setters: () => ({prop: dart.fnType(dart.void, [core.String])}),
     methods: () => ({
       untyped_method: dart.fnType(dart.dynamic, [dart.dynamic, dart.dynamic]),
-      pass: dart.fnType(T, [T]),
+      pass: dart.fnType(T, [core.Object]),
       typed_method: dart.fnType(core.String, [closure.Foo, core.List, core.int, core.num, core.double, core.bool, core.String, js.JsArray, js.JsObject, js.JsFunction]),
       optional_params: dart.fnType(dart.dynamic, [dart.dynamic], [dart.dynamic, core.int]),
       nullary_method: dart.fnType(dart.dynamic, []),
       function_params: dart.fnType(dart.dynamic, [dynamic__Toint(), dynamic__Todynamic(), closure.Callback]),
       run: dart.fnType(dart.dynamic, [core.List, core.String, StringToList(), FnToListOfint()], {h: MapOfMap$Map()})
     }),
     statics: () => ({named_params: dart.fnType(dart.dynamic, [dart.dynamic], {b: dart.dynamic, c: core.int})}),
     names: ['named_params']
   });
   return Foo;
@@ skipping 36 matching lines @@
     return "abc";
   },
   get some_top_level_var() {
     return "abc";
   },
   set some_top_level_var(_) {}
 });
 dart.trackLibraries("closure", {
   "closure.dart": closure
 }, null);