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

lib/runtime/dart/_foreign_helper.js

 var _foreign_helper;
 (function(exports) {
   'use strict';
   // Function JS: (String, String, [dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic]) → dynamic
   function JS(typeDescription, codeTemplate, arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9, arg10, arg11) {
     if (arg0 === void 0)
       arg0 = null;
     if (arg1 === void 0)
       arg1 = null;
     if (arg2 === void 0)
@@ skipping 93 matching lines @@
   // Function JS_EMBEDDED_GLOBAL: (String, String) → dynamic
   function JS_EMBEDDED_GLOBAL(typeDescription, name) {
   }
   // Function JS_GET_FLAG: (String) → bool
   function JS_GET_FLAG(name) {
   }
   // Function JS_EFFECT: (Function) → void
   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
   function JS_STRING_CONCAT(a, b) {
     return a + b;
   }
   // Exports:
   exports.JS = JS;
   exports.JS_CURRENT_ISOLATE_CONTEXT = JS_CURRENT_ISOLATE_CONTEXT;
   exports.IsolateContext = IsolateContext;
@@ skipping 19 matching lines @@
   exports.JS_FUNCTION_TYPE_REQUIRED_PARAMETERS_TAG = JS_FUNCTION_TYPE_REQUIRED_PARAMETERS_TAG;
   exports.JS_FUNCTION_TYPE_OPTIONAL_PARAMETERS_TAG = JS_FUNCTION_TYPE_OPTIONAL_PARAMETERS_TAG;
   exports.JS_FUNCTION_TYPE_NAMED_PARAMETERS_TAG = JS_FUNCTION_TYPE_NAMED_PARAMETERS_TAG;
   exports.JS_GET_NAME = JS_GET_NAME;
   exports.JS_EMBEDDED_GLOBAL = JS_EMBEDDED_GLOBAL;
   exports.JS_GET_FLAG = JS_GET_FLAG;
   exports.JS_EFFECT = JS_EFFECT;
   exports.JS_CONST = JS_CONST;
   exports.JS_STRING_CONCAT = JS_STRING_CONCAT;
 })(_foreign_helper || (_foreign_helper = {}));