No warning shown for duplicate attributes such as 'class'

This component template doesn't show any warning while editing:

<material-button class="use-session-btn" class="blue">Use</material-button>

During compile time we see this error:
ParseErrorLevel.FATAL: Found multiple attributes with the same name: class.

[MichaelRFairhurst]

Thanks for filing!

You're right. Our old parser would reject this before we even started to analyze, so we never implemented this. On the new parser, it'd be a really good & easy thing to catch!