Use signature inheritance when method types are unchanged

test/codegen/expect/cascade.js

 var cascade = dart.defineLibrary(cascade, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   class A extends core.Object {
     A() {
       this.x = null;
     }
   }
-  dart.setSignature(A, {});
   function test_closure_with_mutate() {
     let a = new A();
     a.x = dart.fn(() => {
       core.print("hi");
       a = null;
     });
     let _ = a;
     dart.dcall(_.x);
     dart.dcall(_.x);
     core.print(a);
@@ skipping 62 matching lines @@
       return a;
     })());
   }
   dart.fn(test_increment, dart.void, []);
   let Base$ = dart.generic(function(T) {
     class Base extends core.Object {
       Base() {
         this.x = dart.setType([], core.List$(T));
       }
     }
-    dart.setSignature(Base, {});
     return Base;
   });
   let Base = Base$();
   class Foo extends Base$(core.int) {
     Foo() {
       super.Base();
     }
     test_final_field_generic(t) {
       this.x[core.$add](1);
       this.x[core.$add](2);
@@ skipping 11 matching lines @@
   exports.test_mutate_inside_cascade = test_mutate_inside_cascade;
   exports.test_mutate_outside_cascade = test_mutate_outside_cascade;
   exports.test_VariableDeclaration_single = test_VariableDeclaration_single;
   exports.test_VariableDeclaration_last = test_VariableDeclaration_last;
   exports.test_VariableDeclaration_first = test_VariableDeclaration_first;
   exports.test_increment = test_increment;
   exports.Base$ = Base$;
   exports.Base = Base;
   exports.Foo = Foo;
 })(cascade, core);