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

test/codegen/expect/temps.js

 var temps;
 (function(exports) {
   'use strict';
   let _x = Symbol('_x');
   let __x = Symbol('__x');
   let _function = Symbol('_function');
   class FormalCollision extends core.Object {
     FormalCollision(x, _x$, func) {
-      this[_x] = x;
-      this[__x] = _x$;
-      this[_function] = func;
+      dart.initField(FormalCollision, this, _x, x);
+      dart.initField(FormalCollision, this, __x, _x$);
+      dart.initField(FormalCollision, this, _function, func);
     }
   }
   // Function main: () → dynamic
   function main() {
     core.print(new FormalCollision(1, 2, x => x));
   }
   // Exports:
   exports.FormalCollision = FormalCollision;
   exports.main = main;
 })(temps || (temps = {}));