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

test/codegen/expect/opassign.js

 var opassign;
 (function(exports) {
   'use strict';
   dart.copyProperties(exports, {
     get index() {
       core.print('called "index" getter');
       return 0;
     }
   });
   dart.defineLazyProperties(exports, {
     get _foo() {
       return new Foo();
     }
   });
   dart.copyProperties(exports, {
     get foo() {
       core.print('called "foo" getter');
       return exports._foo;
     }
   });
+  let x = Symbol('x');
   class Foo extends core.Object {
     Foo() {
-      this.x = 100;
+      this[x] = 100;
+    }
+    get x() {
+      return this[x];
+    }
+    set x(value) {
+      this[x] = value;
     }
   }
   // Function main: () → dynamic
   function main() {
     let f = dart.map([0, 40]);
     core.print('should only call "index" 2 times:');
     let i = exports.index;
     f.set(i, dart.dsend(f.get(i), '+', 1));
     forcePostfix((() => {
       let i = exports.index, x = f.get(i);
@@ skipping 13 matching lines @@
     f.set(i$, dart.dsend(f.get(i$), '+', f.get(exports.index)));
   }
   // Function forcePostfix: (dynamic) → dynamic
   function forcePostfix(x) {
   }
   // Exports:
   exports.Foo = Foo;
   exports.main = main;
   exports.forcePostfix = forcePostfix;
 })(opassign || (opassign = {}));