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;
+  }
+  let x = Symbol('x');
+  class Base extends core.Object {
+    Base() {
+      this[x] = init('Base.x');
+    }
+    get x() {
+      return this[x];
+    }
+    set x(value) {
+      this[x] = value;
+    }
+  }
+  let x$ = Symbol('x');
+  class Mixin1 extends core.Object {
+    Mixin1() {
+      this[x$] = init('Mixin1.x');
+    }
+    get x() {
+      return this[x$];
+    }
+    set x(value) {
+      this[x$] = value;
+    }
+  }
+  let x$0 = Symbol('x');
+  class Mixin2 extends core.Object {
+    Mixin2() {
+      this[x$0] = init('Mixin2.x');
+    }
+    get x() {
+      return this[x$0];
+    }
+    set x(value) {
+      this[x$0] = value;
+    }
+  }
+  let x$1 = Symbol('x');
+  class Derived extends dart.mixin(Base, Mixin1, Mixin2) {
+    Derived() {
+      this[x$1] = init('Derived.x');
+      super.Base();
+    }
+    get x() {
+      return this[x$1];
+    }
+    set x(value) {
+      this[x$1] = value;
+    }
+  }
+  class _Base extends core.Object {
+    _Base() {
+      this.x = init('_Base.x');
+    }
+  }
+  let _x = Symbol('_x');
+  let _x$ = Symbol('_x');
+  class _Mixin1 extends core.Object {
+    _Mixin1() {
+      this[_x] = init('_Mixin1.x');
+    }
+    get [_x$]() {
+      return this[_x];
+    }
+    set [_x$](value) {
+      this[_x] = value;
+    }
+  }
+  let _x$0 = Symbol('_x');
+  class _Mixin2 extends core.Object {
+    _Mixin2() {
+      this[_x$0] = init('_Mixin2.x');
+    }
+    get [_x$]() {
+      return this[_x$0];
+    }
+    set [_x$](value) {
+      this[_x$0] = value;
+    }
+  }
+  let _x$1 = Symbol('_x');
+  class _Derived extends dart.mixin(_Base, _Mixin1, _Mixin2) {
+    _Derived() {
+      this[_x$1] = init('_Derived.x');
+      super._Base();
+    }
+    get [_x$]() {
+      return this[_x$1];
+    }
+    set [_x$](value) {
+      this[_x$1] = value;
+    }
+  }
+  // Function main: () → dynamic
+  function main() {
+    core.print('Creating Derived');
+    core.print('Derived.x == ' + dart.notNull(new Derived().x));
+    core.print('Creating _Derived');
+    core.print('_Derived.x == ' + dart.notNull(new _Derived()[_x$]));
+  }
+  // Exports:
+  exports.init = init;
+  exports.Base = Base;
+  exports.Mixin1 = Mixin1;
+  exports.Mixin2 = Mixin2;
+  exports.Derived = Derived;
+  exports.main = main;
+})(fields || (fields = {}));