Use signature inheritance when method types are unchanged

test/codegen/expect/names.js

 var names = dart.defineLibrary(names, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   exports.exports = 42;
   let _foo$ = Symbol('_foo');
   class Foo extends core.Object {
     [_foo$]() {
       return 123;
     }
@@ skipping 12 matching lines @@
     static callee() {
       return null;
     }
   }
   dart.defineNamedConstructor(Frame, 'caller');
   dart.setSignature(Frame, {
     statics: () => ({callee: dart.functionType(dart.dynamic, [])}),
     names: ['callee']
   });
   class Frame2 extends core.Object {}
-  dart.setSignature(Frame2, {});
   dart.defineLazyProperties(Frame2, {
     get caller() {
       return 100;
     },
     set caller(_) {},
     get arguments() {
       return 200;
     },
     set arguments(_) {}
   });
   function main() {
     core.print(exports.exports);
     core.print(new Foo()[_foo$]());
     core.print(_foo());
     core.print(new Frame.caller([1, 2, 3]));
     let eval$ = Frame.callee;
     core.print(eval$);
     core.print(dart.notNull(Frame2.caller) + dart.notNull(Frame2.arguments));
   }
   dart.fn(main);
   // Exports:
   exports.Foo = Foo;
   exports.Frame = Frame;
   exports.Frame2 = Frame2;
   exports.main = main;
 })(names, core);