VM: The loader's extension of Dart_LibraryTag invokes undefined behavior

[rmacnak-google]

The loader uses values for Dart_LibraryTag that aren't declared in the enum in dart_api.h. Using such values is undefined behavior.

[johnmccutchan]

I'm not sure I understand the issue here. The tag handler defines a set of operations and for convenience the service isolate's loader uses the same tag values as the tag handler but then adds more loader-specific tag values. These loader-specific tag values are never seen by the tag handler.