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

test/codegen/expect/fieldtest.js

 var fieldtest;
 (function(exports) {
   'use strict';
   class A extends core.Object {
     A() {
-      this.x = 42;
+      dart.initField(A, this, 'x', 42);
     }
   }
   let B$ = dart.generic(function(T) {
     class B extends core.Object {
       B() {
-        this.x = null;
-        this.y = null;
-        this.z = null;
+        dart.initField(B, this, 'x', null);
+        dart.initField(B, this, 'y', null);
+        dart.initField(B, this, 'z', null);
       }
     }
     return B;
   });
   let B = B$();
   // Function foo: (A) → int
   function foo(a) {
     core.print(a.x);
     return a.x;
   }
@@ skipping 37 matching lines @@
   // Exports:
   exports.A = A;
   exports.B$ = B$;
   exports.B = B;
   exports.foo = foo;
   exports.bar = bar;
   exports.baz = baz;
   exports.compute = compute;
   exports.main = main;
 })(fieldtest || (fieldtest = {}));