Move generated files to gen/.

test/codegen/expect/functions.js

-dart_library.library('functions', null, /* Imports */[
-  'dart_sdk'
-], function(exports, dart_sdk) {
-  'use strict';
-  const core = dart_sdk.core;
-  const dart = dart_sdk.dart;
-  const dartx = dart_sdk.dartx;
-  const functions = Object.create(null);
-  functions.bootstrap = function() {
-    return dart.list([new functions.Foo()], functions.Foo);
-  };
-  dart.lazyFn(functions.bootstrap, () => [core.List$(functions.Foo), []]);
-  functions.A2B$ = dart.generic((A, B) => {
-    const A2B = dart.typedef('A2B', () => dart.functionType(B, [A]));
-    return A2B;
-  });
-  functions.A2B = functions.A2B$();
-  functions.id = function(f) {
-    return f;
-  };
-  dart.lazyFn(functions.id, () => [functions.A2B$(functions.Foo, functions.Foo), [functions.A2B$(functions.Foo, functions.Foo)]]);
-  functions.Foo = class Foo extends core.Object {};
-  functions.main = function() {
-    core.print(functions.bootstrap()[dartx.get](0));
-  };
-  dart.fn(functions.main, dart.void, []);
-  // Exports:
-  exports.functions = functions;
-});