dart2js cps: Ensure JSArray is emitted when we see a type cast.

tests/standalone/standalone.status

 # Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # WARNING:
 # Tests using the multitest feature where failure is expected should *also* be
 # listed in tests/lib/analyzer/analyze_tests.status without the "standalone"
 # prefix.
 
 package/invalid_uri_test: Fail, OK # CompileTimeErrors intentionally
@@ skipping 163 matching lines @@
 io/file_write_only_test: Crash # (main()async{asyncSt...  cannot handle async/sync*/async* functions
 io/http_bind_test: Crash # (testBindShared(Stri...  cannot handle async/sync*/async* functions
 io/http_parser_test : RuntimeError # TypeError: receiver.get$_nums is not a function
 io/https_bad_certificate_test: Crash # (main()async{var cli...  cannot handle async/sync*/async* functions
 io/issue_22636_test: Crash # (test()async{server=...  cannot handle async/sync*/async* functions
 io/issue_22637_test: Crash # (test()async{server=...  cannot handle async/sync*/async* functions
 io/observatory_test: Crash # Invalid argument(s)
 io/socket_bind_test: Crash # (testListenCloseList...  cannot handle async/sync*/async* functions
 io/socket_source_address_test: Crash # (Future testConnect(...  cannot handle async/sync*/async* functions
 priority_queue_stress_test: RuntimeError # receiver.get$_collection$_nums is not a function
-slowpath_safepoints_test: RuntimeError # Cannot read property 'prototype' of undefined
 typed_array_test: RuntimeError # receiver.get$_collection$_nums is not a function