Use signature inheritance when method types are unchanged

test/codegen/expect/temps.js

 var temps = dart.defineLibrary(temps, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   let _x = Symbol('_x');
   let __x = Symbol('__x');
   let _function = Symbol('_function');
   class FormalCollision extends core.Object {
     FormalCollision(x, _x$, func) {
       this[_x] = x;
       this[__x] = _x$;
       this[_function] = func;
     }
   }
-  dart.setSignature(FormalCollision, {});
   let _opt = Symbol('_opt');
   class OptionalArg extends core.Object {
     OptionalArg(opt) {
       if (opt === void 0)
         opt = 123;
       this[_opt] = opt;
     }
     named(opts) {
       let opt = opts && '_opt' in opts ? opts._opt : 456;
       this[_opt] = opt;
     }
   }
   dart.defineNamedConstructor(OptionalArg, 'named');
-  dart.setSignature(OptionalArg, {});
   function main() {
     core.print(new FormalCollision(1, 2, dart.fn(x => x)));
     core.print(new OptionalArg()[_opt]);
     core.print(new OptionalArg.named()[_opt]);
   }
   dart.fn(main);
   // Exports:
   exports.FormalCollision = FormalCollision;
   exports.OptionalArg = OptionalArg;
   exports.main = main;
 })(temps, core);