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

test/codegen/expect/fieldtest.js

 var fieldtest;
 (function(exports) {
   'use strict';
+  let x = Symbol('x');
   class A extends core.Object {
     A() {
-      this.x = 42;
+      this[x] = 42;
+    }
+    get x() {
+      return this[x];
+    }
+    set x(value) {
+      this[x] = value;
     }
   }
+  let x$ = Symbol('x');
+  let y = Symbol('y');
+  let z = Symbol('z');
   let B$ = dart.generic(function(T) {
     class B extends core.Object {
       B() {
-        this.x = null;
-        this.y = null;
-        this.z = null;
+        this[x$] = null;
+        this[y] = null;
+        this[z] = null;
+      }
+      get x() {
+        return this[x$];
+      }
+      set x(value) {
+        this[x$] = value;
+      }
+      get y() {
+        return this[y];
+      }
+      set y(value) {
+        this[y] = value;
+      }
+      get z() {
+        return this[z];
+      }
+      set z(value) {
+        this[z] = value;
       }
     }
     return B;
   });
   let B = B$();
   // 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 = {}));