Deal with "classes" that don't belong to any library. Addresses issue 12231.

runtime/lib/mirrors.cc

Index: runtime/lib/mirrors.cc
diff --git a/runtime/lib/mirrors.cc b/runtime/lib/mirrors.cc
index 176aecb5854b7159b4443a9ddf963201a2c6407e..874915f014d0e29471272f1eac0dc59dd4f5c090 100644
--- a/runtime/lib/mirrors.cc
+++ b/runtime/lib/mirrors.cc
@@ -362,7 +362,13 @@ DEFINE_NATIVE_ENTRY(ClassMirror_name, 1) {
 DEFINE_NATIVE_ENTRY(ClassMirror_library, 1) {
   GET_NON_NULL_NATIVE_ARGUMENT(MirrorReference, ref, arguments->NativeArgAt(0));
   const Class& klass = Class::Handle(ref.GetClassReferent());
-  return CreateLibraryMirror(Library::Handle(klass.library()));
+  const Library& library = Library::Handle(klass.library());
+  // TODO(rmacnak): Revisit when we decide what to do about
+  // reflectClass(dynamic).
+  if (library.IsNull()) {
+    return Instance::null();
+  }
+  return CreateLibraryMirror(library);
 }