implement private members, fixes #74

test/codegen/expect/methods/methods.js

 var methods;
 (function(exports) {
   'use strict';
+  let _c = Symbol('_c');
   class A extends dart.Object {
     A() {
-      this._c = 3;
+      this[_c] = 3;
     }
     x() {
       return 42;
     }
     y(a) {
       return a;
     }
     z(b) {
       if (b === void 0)
         b = null;
@@ skipping 10 matching lines @@
     }
     ww(a, opt$) {
       let b = opt$.b === void 0 ? 0 : opt$.b;
       return a + b;
     }
     get a() {
       return this.x();
     }
     set b(b) {}
     get c() {
-      return this._c;
+      return this[_c];
     }
     set c(c) {
-      this._c = c;
+      this[_c] = c;
     }
   }
   class Bar extends dart.Object {
     call(x) {
       return core.print(`hello from ${x}`);
     }
   }
   class Foo extends dart.Object {
     Foo() {
       this.bar = new Bar();
     }
   }
   // Function test: () → dynamic
   function test() {
     let f = new Foo();
     dart.dinvoke(f, 'bar', "Bar's call method!");
   }
   // Exports:
   exports.A = A;
   exports.Bar = Bar;
   exports.Foo = Foo;
   exports.test = test;
 })(methods || (methods = {}));