Use signature inheritance when method types are unchanged

test/codegen/expect/methods.js

 var methods = dart.defineLibrary(methods, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   let _c = Symbol('_c');
   class A extends core.Object {
     A() {
       this[_c] = 3;
     }
     x() {
@@ skipping 47 matching lines @@
     }
   }
   dart.setSignature(Bar, {
     methods: () => ({call: dart.functionType(dart.dynamic, [dart.dynamic])})
   });
   class Foo extends core.Object {
     Foo() {
       this.bar = new Bar();
     }
   }
-  dart.setSignature(Foo, {});
   function test() {
     let f = new Foo();
     dart.dcall(f.bar, "Bar's call method!");
     let a = new A();
     let g = dart.bind(a, 'x');
     let aa = new A();
     let h = dart.dload(aa, 'x');
   }
   dart.fn(test);
   // Exports:
   exports.A = A;
   exports.Bar = Bar;
   exports.Foo = Foo;
   exports.test = test;
 })(methods, core);