ThemeIcons used by vscode extensions do not load correctly

[msujew]

Bug Description:

See title. I assume this regression was introduced by #9436 as the instanceof check fails here:

theia/packages/plugin-ext/src/plugin/tree/tree-views.ts

Line 372 in 7442150

} else if (iconPath instanceof ThemeIcon) {

Steps to Reproduce:

1. Install the npm extension (built-in)
2. Open the npm view
3. Notice the missing icon

Additional Information

• Operating System: Ubuntu (Gitpod)
• Theia Version: a7d63a0

[tsmaeder]

Just for my understanding: why is the instanceof failing?

[msujew]

@tsmaeder As far as I understand the different ES versions, ES5 classes work quite differently from classes in newer ES versions. That's why the @es5Compatibility annotation is needed, so that extensions compiled with es5 can still use them without issues.

That however introduces a problem related to the TypeScript compiler which tries to emit newer ES2017 instanceof calls for classes compiled to ES5, which just don't work. The prototype chain seems to be completely different, as Object.getProtoTypeOf(iconPath) === ThemeIcon.prototype returns false as well, even when the debugger shows the prototype of iconPath to be ThemeIcon.