Move generated files to gen/.

test/codegen/expect/es6_modules.js

-export const es6_modules = Object.create(null);
-import { core, dart, dartx } from 'dart_sdk';
-es6_modules.Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
-es6_modules.A = class A extends core.Object {};
-es6_modules._A = class _A extends core.Object {};
-es6_modules.B$ = dart.generic(T => {
-  class B extends core.Object {}
-  return B;
-});
-es6_modules.B = es6_modules.B$();
-es6_modules._B$ = dart.generic(T => {
-  class _B extends core.Object {}
-  return _B;
-});
-es6_modules._B = es6_modules._B$();
-es6_modules.f = function() {
-};
-dart.fn(es6_modules.f);
-es6_modules._f = function() {
-};
-dart.fn(es6_modules._f);
-es6_modules.constant = "abc";
-es6_modules.finalConstant = "abc";
-dart.defineLazy(es6_modules, {
-  get lazy() {
-    return dart.fn(() => {
-      core.print('lazy');
-      return "abc";
-    }, core.String, [])();
-  }
-});
-es6_modules.mutable = "abc";
-dart.defineLazy(es6_modules, {
-  get lazyMutable() {
-    return dart.fn(() => {
-      core.print('lazyMutable');
-      return "abc";
-    }, core.String, [])();
-  },
-  set lazyMutable(_) {}
-});