Regression: (WebAssembly-based) Blazor websites no longer load

Originator:chucker23n
Number:rdar://50400451 Date Originated:
Status: Resolved:
Product:macOS + SDK Safari Technology Preview Product Version:
Classification:Serious Bug Reproducible:Always
 
Summary:
Websites using Blazor (http://blazor.net) with the WebAssembly-based client-side hosting model (https://docs.microsoft.com/en-us/aspnet/core/blazor/hosting-models?view=aspnetcore-3.0#client-side) no longer work with Safari Technology Preview. They still work with the current release build Version 12.1.1 (14607.2.4.1).

Steps to Reproduce:
Go to a Blazor site, such as https://www.iheartblazor.com.

Expected Results:
It should load for a few seconds, then show up.

Actual Results:
It gets stuck loading, because an exception occurs. The Web Inspector Console shows:

[Error] Unhandled Promise Rejection: TypeError: 'arguments', 'callee', and 'caller' cannot be accessed in this context.
	(anonymous function) (mono.js:1:226214)
	run (mono.js:1:226214)
	runCaller (mono.js:1:226118)
	removeRunDependency (mono.js:1:18052)
	receiveInstance (mono.js:1:20407)
	receiveInstantiatedSource (mono.js:1:20696)
	promiseReactionJob

Version/Build:
Release 81 (Safari 12.2, WebKit 14608.1.19.5)

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