Add a runtime flag to control availability of dart:mirrors.

runtime/vm/bootstrap_natives.cc

 // Copyright (c) 2012, 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.
 
 #include "vm/bootstrap.h"
 
 #include "include/dart_api.h"
 
 #include "vm/bootstrap_natives.h"
 #include "vm/dart_api_impl.h"
@@ skipping 98 matching lines @@
   ASSERT(!library.IsNull());
   library.set_native_entry_resolver(resolver);
   library.set_native_entry_symbol_resolver(symbol_resolver);
 
   library = Library::MathLibrary();
   ASSERT(!library.IsNull());
   library.set_native_entry_resolver(resolver);
   library.set_native_entry_symbol_resolver(symbol_resolver);
 
   library = Library::MirrorsLibrary();
-  ASSERT(!library.IsNull());
-  library.set_native_entry_resolver(resolver);
-  library.set_native_entry_symbol_resolver(symbol_resolver);
+  if (!library.IsNull()) {
+    library.set_native_entry_resolver(resolver);
+    library.set_native_entry_symbol_resolver(symbol_resolver);
+  }
 
   library = Library::ProfilerLibrary();
   ASSERT(!library.IsNull());
   library.set_native_entry_resolver(resolver);
   library.set_native_entry_symbol_resolver(symbol_resolver);
 
   library = Library::TypedDataLibrary();
   ASSERT(!library.IsNull());
   library.set_native_entry_resolver(resolver);
   library.set_native_entry_symbol_resolver(symbol_resolver);
 }
 
 
 bool Bootstrap::IsBootstapResolver(Dart_NativeEntryResolver resolver) {
   return (resolver ==
           reinterpret_cast<Dart_NativeEntryResolver>(BootstrapNatives::Lookup));
 }
 
 }  // namespace dart