Move generated files to gen/.

test/codegen/expect/js/js.js

-dart_library.library('js', null, /* Imports */[
-  'dart_sdk'
-], function(exports, dart_sdk) {
-  'use strict';
-  const core = dart_sdk.core;
-  const js = dart_sdk.js;
-  const dart = dart_sdk.dart;
-  const dartx = dart_sdk.dartx;
-  const js$ = Object.create(null);
-  js$.JS = class JS extends core.Object {
-    new(name) {
-      if (name === void 0) name = null;
-      this.name = name;
-    }
-  };
-  dart.setSignature(js$.JS, {
-    constructors: () => ({new: [js$.JS, [], [core.String]]})
-  });
-  js$._Anonymous = class _Anonymous extends core.Object {
-    new() {
-    }
-  };
-  dart.setSignature(js$._Anonymous, {
-    constructors: () => ({new: [js$._Anonymous, []]})
-  });
-  js$.anonymous = dart.const(new js$._Anonymous());
-  js$.allowInteropCaptureThis = js.allowInteropCaptureThis;
-  js$.allowInterop = js.allowInterop;
-  // Exports:
-  exports.js = js$;
-});