Use signature inheritance when method types are unchanged

test/codegen/expect/opassign.js

 var opassign = dart.defineLibrary(opassign, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   dart.copyProperties(exports, {
     get index() {
       core.print('called "index" getter');
       return 0;
     }
   });
   dart.defineLazyProperties(exports, {
     get _foo() {
       return new Foo();
     }
   });
   dart.copyProperties(exports, {
     get foo() {
       core.print('called "foo" getter');
       return exports._foo;
     }
   });
   class Foo extends core.Object {
     Foo() {
       this.x = 100;
     }
   }
-  dart.setSignature(Foo, {});
   function main() {
     let f = dart.map([0, 40]);
     core.print('should only call "index" 2 times:');
     let i = exports.index;
     f.set(i, dart.dsend(f.get(i), '+', 1));
     forcePostfix((() => {
       let i = exports.index, x = f.get(i);
       f.set(i, dart.dsend(x, '+', 1));
       return x;
     })());
@@ skipping 11 matching lines @@
   }
   dart.fn(main);
   function forcePostfix(x) {
   }
   dart.fn(forcePostfix);
   // Exports:
   exports.Foo = Foo;
   exports.main = main;
   exports.forcePostfix = forcePostfix;
 })(opassign, core);