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

test/codegen/expect/methods.js

 var methods;
 (function(exports) {
   'use strict';
   let _c = Symbol('_c');
   class A extends core.Object {
     A() {
-      this[_c] = 3;
+      dart.initField(A, this, _c, 3);
     }
     x() {
       return 42;
     }
     y(a) {
       return a;
     }
     z(b) {
       if (b === void 0)
         b = null;
@@ skipping 23 matching lines @@
       this[_c] = c;
     }
   }
   class Bar extends core.Object {
     call(x) {
       return core.print(`hello from ${x}`);
     }
   }
   class Foo extends core.Object {
     Foo() {
-      this.bar = new Bar();
+      dart.initField(Foo, this, 'bar', new Bar());
     }
   }
   // Function test: () → dynamic
   function test() {
     let f = new Foo();
     dart.dsend(f, 'bar', "Bar's call method!");
   }
   // Exports:
   exports.A = A;
   exports.Bar = Bar;
   exports.Foo = Foo;
   exports.test = test;
 })(methods || (methods = {}));