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

test/codegen/expect/fields.js

+var fields;
+(function(exports) {
+  'use strict';
+  // Function init: (dynamic) → dynamic
+  function init(x) {
+    core.print(x);
+    return x;
+  }
+  class Base extends core.Object {
+    Base() {
+      dart.initField(Base, this, 'x', init('Base.x'));
+    }
+  }
+  dart.virtualField(Base, 'x');
+  class Mixin1 extends core.Object {
+    Mixin1() {
+      dart.initField(Mixin1, this, 'x', init('Mixin1.x'));
+    }
+  }
+  dart.virtualField(Mixin1, 'x');
+  class Mixin2 extends core.Object {
+    Mixin2() {
+      dart.initField(Mixin2, this, 'x', init('Mixin2.x'));
+    }
+  }
+  dart.virtualField(Mixin2, 'x');
+  class Derived extends dart.mixin(Base, Mixin1, Mixin2) {
+    Derived() {
+      dart.initField(Derived, this, 'x', init('Derived.x'));
+      super.Base();
+    }
+  }
+  class _Base extends core.Object {
+    _Base() {
+      this.x = init('_Base.x');
+    }
+  }
+  let _x = Symbol('_x');
+  class _Mixin1 extends core.Object {
+    _Mixin1() {
+      dart.initField(_Mixin1, this, _x, init('_Mixin1.x'));
+    }
+  }
+  dart.virtualField(_Mixin1, _x);
+  class _Mixin2 extends core.Object {
+    _Mixin2() {
+      dart.initField(_Mixin2, this, _x, init('_Mixin2.x'));
+    }
+  }
+  dart.virtualField(_Mixin2, _x);
+  class _Derived extends dart.mixin(_Base, _Mixin1, _Mixin2) {
+    _Derived() {
+      this[_x] = init('_Derived.x');
+      super._Base();
+    }
+  }
+  // Function main: () → dynamic
+  function main() {
+    core.print('Creating Derived');
+    core.print('Derived.x == ' + dart.notNull(new Derived().x));
+    core.print('Creating _Derived');
+    core.print('_Derived.x == ' + dart.notNull(new _Derived()[_x]));
+  }
+  // Exports:
+  exports.init = init;
+  exports.Base = Base;
+  exports.Mixin1 = Mixin1;
+  exports.Mixin2 = Mixin2;
+  exports.Derived = Derived;
+  exports.main = main;
+})(fields || (fields = {}));