implement mixins in subtype checks, more codegen fixes

test/codegen/expect/fieldtest.js

 var fieldtest;
 (function(exports) {
   'use strict';
   class A extends core.Object {
     A() {
       this.x = 42;
     }
   }
   let B$ = dart.generic(function(T) {
     class B extends core.Object {
       B() {
         this.x = null;
         this.y = null;
         this.z = null;
       }
     }
     return B;
   });
-  let B = B$(dart.dynamic);
+  let B = B$();
   // Function foo: (A) → int
   function foo(a) {
     core.print(a.x);
     return a.x;
   }
   // Function bar: (dynamic) → int
   function bar(a) {
     core.print(dart.dload(a, 'x'));
     return dart.as(dart.dload(a, 'x'), core.int);
   }
@@ skipping 24 matching lines @@
   });
   // Function main: () → void
   function main() {
     let a = new A();
     foo(a);
     bar(a);
     core.print(baz(a));
   }
   // Exports:
   exports.A = A;
+  exports.B$ = B$;
   exports.B = B;
-  exports.B$ = B$;
   exports.foo = foo;
   exports.bar = bar;
   exports.baz = baz;
   exports.compute = compute;
   exports.main = main;
 })(fieldtest || (fieldtest = {}));