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

test/codegen/expect/names.js

 var names;
 (function(exports) {
   'use strict';
   exports.exports = 42;
   let _foo$ = Symbol('_foo');
   class Foo extends core.Object {
     [_foo$]() {
       return 123;
     }
   }
   // Function _foo: () → dynamic
   function _foo() {
     return 456;
   }
   class Frame extends core.Object {
     ['caller*'](arguments$) {
-      this.arguments = arguments$;
+      dart.initField(Frame, this, 'arguments', arguments$);
     }
     static ['callee*']() {
       return null;
     }
   }
   dart.defineNamedConstructor(Frame, 'caller*');
   // Function main: () → dynamic
   function main() {
     core.print(exports.exports);
     core.print(new Foo()[_foo$]());
     core.print(_foo());
     core.print(new Frame.caller(new core.List.from([1, 2, 3])));
     let eval$ = Frame.callee;
     core.print(eval$);
   }
   // Exports:
   exports.Foo = Foo;
   exports.Frame = Frame;
   exports.main = main;
 })(names || (names = {}));