Type check binary expressions

test/codegen/expect/fieldtest.js

 var fieldtest = dart.defineLibrary(fieldtest, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   class A extends core.Object {
     A() {
       this.x = 42;
     }
   }
   dart.setSignature(A, {});
@@ skipping 55 matching lines @@
       this.foo = 2;
       this.bar = 3;
     }
   }
   dart.virtualField(Derived, 'foo');
   dart.setSignature(Derived, {});
   let Generic$ = dart.generic(function(T) {
     class Generic extends core.Object {
       foo(t) {
         dart.as(t, T);
-        return core.print(dart.notNull(Generic$().bar) + dart.notNull(t));
+        return core.print(dart.notNull(Generic$().bar) + dart.notNull(dart.as(t, core.String)));
       }
     }
     dart.setSignature(Generic, {
       methods: () => ({foo: dart.functionType(dart.dynamic, [T])})
     });
     return Generic;
   });
   let Generic = Generic$();
   Generic.bar = 'hello';
   function main() {
@@ skipping 11 matching lines @@
   exports.foo = foo;
   exports.bar = bar;
   exports.baz = baz;
   exports.compute = compute;
   exports.BaseWithGetter = BaseWithGetter;
   exports.Derived = Derived;
   exports.Generic$ = Generic$;
   exports.Generic = Generic;
   exports.main = main;
 })(fieldtest, core);