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

test/codegen/expect/cascade.js

 var cascade;
 (function(exports) {
   'use strict';
+  let x = Symbol('x');
   class A extends core.Object {
     A() {
-      this.x = null;
+      this[x] = null;
+    }
+    get x() {
+      return this[x];
+    }
+    set x(value) {
+      this[x] = value;
     }
   }
   // Function test_closure_with_mutate: () → void
   function test_closure_with_mutate() {
     let a = new A();
     a.x = () => {
       core.print("hi");
       a = null;
     };
     let _ = a;
@@ skipping 58 matching lines @@
   }
   // Function test_increment: () → void
   function test_increment() {
     let a = new A();
     let y = ((() => {
       a.x = dart.dsend(a.x, '+', 1);
       a.x = dart.dsend(a.x, '-', 1);
       return a;
     })());
   }
+  let x$ = Symbol('x');
   let Base$ = dart.generic(function(T) {
     class Base extends core.Object {
       Base() {
-        this.x = new core.List$(T).from([]);
+        this[x$] = new core.List$(T).from([]);
+      }
+      get x() {
+        return this[x$];
       }
     }
     return Base;
   });
   let Base = Base$();
   class Foo extends Base$(core.int) {
     test_final_field_generic(t) {
       this.x[core.$add](1);
       this.x[core.$add](2);
       this.x[core.$add](3);
       this.x[core.$add](4);
     }
   }
   // Exports:
   exports.A = A;
   exports.test_closure_with_mutate = test_closure_with_mutate;
   exports.test_closure_without_mutate = test_closure_without_mutate;
   exports.test_mutate_inside_cascade = test_mutate_inside_cascade;
   exports.test_mutate_outside_cascade = test_mutate_outside_cascade;
   exports.test_VariableDeclaration_single = test_VariableDeclaration_single;
   exports.test_VariableDeclaration_last = test_VariableDeclaration_last;
   exports.test_VariableDeclaration_first = test_VariableDeclaration_first;
   exports.test_increment = test_increment;
   exports.Base$ = Base$;
   exports.Base = Base;
   exports.Foo = Foo;
 })(cascade || (cascade = {}));