fixes #52, fields shadowing getters/setters or other fields

lib/runtime/dart/_foreign_helper.js

Index: lib/runtime/dart/_foreign_helper.js
diff --git a/lib/runtime/dart/_foreign_helper.js b/lib/runtime/dart/_foreign_helper.js
index e92813ee44df324c452cabe4474a9a3993dc6659..90b68779e73734f98b44fa64d39af16a051f43ff 100644
--- a/lib/runtime/dart/_foreign_helper.js
+++ b/lib/runtime/dart/_foreign_helper.js
@@ -111,9 +111,13 @@ var _foreign_helper;
   function JS_EFFECT(code) {
     dart.dcall(code, null);
   }
+  let code$ = Symbol('code');
   class JS_CONST extends core.Object {
+    get code() {
+      return this[code$];
+    }
     JS_CONST(code) {
-      this.code = code;
+      this[code$] = code;
     }
   }
   // Function JS_STRING_CONCAT: (String, String) → String