dart2js: fix crash when annotations have syntax errors (fix #23983)

tests/compiler/dart2js_extra/invalid_annotation2_test.dart

+// Copyright (c) 2015, 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.
+
+// Regression test for http://dartbug.com/23893/
+//
+// Because annotations are parsed lazily, dart2js used to crash when an
+// annotation had a syntax error.
+// This checks the same behavior as invalid_annotation_test.dart, except that we
+// use mirrors to trigger the error in the vm. This also triggers the error in
+// dart2js differently.
+
+@MirrorsUsed(targets: const [A])
+import 'dart:mirrors';
+
+@Deprecated("m"
+,                                /// 01: compile-time error
+)
+class A {
+}
+
+main() {
+  reflectClass(A).metadata;
+}