src/vs/workbench/services/extensions/common/abstractExtensionService.ts/ProposedApiController/constructor
class ProposedApiController:	constructor(
		@IWorkbenchEnvironmentService private readonly _environmentService: IWorkbenchEnvironmentService,
		@IProductService productService: IProductService
	) {
		// Make enabled proposed API be lowercase for case insensitive comparison
		this.enableProposedApiFor = (_environmentService.extensionEnabledProposedApi || []).map(id => id.toLowerCase());

		this.enableProposedApiForAll =
			!_environmentService.isBuilt || // always allow proposed API when running out of sources
			(_environmentService.isExtensionDevelopment && productService.quality !== 'stable') || // do not allow proposed API against stable builds when developing an extension
			(this.enableProposedApiFor.length === 0 && Array.isArray(_environmentService.extensionEnabledProposedApi)); // always allow proposed API if --enable-proposed-api is provided without extension ID

		this.productAllowProposedApi = new Set<string>();
		if (isNonEmptyArray(productService.extensionAllowedProposedApi)) {
			productService.extensionAllowedProposedApi.forEach((id) => this.productAllowProposedApi.add(ExtensionIdentifier.toKey(id)));
		}
	}