Move generated files to gen/.

test/codegen/expect/node_modules.js

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