[Bug] Cannot set form attribute of <output>

[srsgores]

🐞 Describe the Bug

Using the <output> element outside of a <form> via the form attribute will cause handlebars to throw the following error:

runtime.js:463 Uncaught (in promise) TypeError: Cannot assign to read only property 'form' of object '#'
at NewElementBuilder.__setProperty (runtime.js:463)
at DefaultDynamicProperty.set (runtime.js:1331)
at NewElementBuilder.setDynamicAttribute (runtime.js:473)
at Object.evaluate (runtime.js:3070)
at AppendOpcodes.evaluate (runtime.js:2008)
at LowLevelVM.evaluateSyscall (runtime.js:4927)
at LowLevelVM.evaluateInner (runtime.js:4883)
at LowLevelVM.evaluateOuter (runtime.js:4875)
at JitVM.next (runtime.js:5824)
at JitVM.execute (runtime.js:5799)

🔬 Minimal Reproduction

Assign the form attribute of an <output> tag to any value:

{{#if this.eventDuration}}
	<output for="{{this.elementId}}-start-time {{this.elementId}}-end-time" form="{{this.elementId}}-form" aria-atomic="true" aria-live="assertive"><span class="visuallyhidden">{{@node.name}} will last </span>{{moment-duration this.eventDuration}}</output>
{{/if}}

🌍 Environment

• Ember: - 3.18.1
• Node.js/npm: - 15.1.0
• OS: - Windows 10
• Browser: - Chromium 85.0.4350.0

[KrishnaRPatel]

I put in a PR to glimmer-vm that should fix this here: glimmerjs/glimmer-vm#1209

[rwjblue]

Thanks @KrishnaRPatel! That is merged and released. I'm working to update glimmer-vm over in #19282 (to fix another bug) and that bump includes the fixes.

[KrishnaRPatel]

@rwjblue Happy to help!