Type check binary expressions

test/codegen/fieldtest.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fieldtest;
 
 class A {
   int x = 42;
 }
 
@@ skipping 28 matching lines @@
 abstract class BaseWithGetter {
   int get foo => 1;
   int get bar;
 }
 class Derived extends BaseWithGetter {
   int foo = 2;
   int bar = 3;
 }
 
 class Generic<T> {
-  foo(T t) => print(bar + t);
+  foo(T t) => print(bar + (t as String));
 
   static String bar = 'hello';
 }
 
 void main() {
   var a = new A();
   foo(a);
   bar(a);
   print(baz(a));
 
   print(new Generic<String>().foo(' world'));
 }