src/vs/platform/extensionManagement/common/extensionsScannerService.ts/AbstractExtensionsScannerService/scanDevSystemExtensions
class AbstractExtensionsScannerService:	private async scanDevSystemExtensions(language: string | undefined, checkControlFile: boolean): Promise<IRelaxedScannedExtension[]> {
		const devSystemExtensionsList = this.environmentService.isBuilt ? [] : this.productService.builtInExtensions;
		if (!devSystemExtensionsList?.length) {
			return [];
		}

		this.logService.trace('Started scanning dev system extensions');
		const builtinExtensionControl = checkControlFile ? await this.getBuiltInExtensionControl() : {};
		const devSystemExtensionsLocations: URI[] = [];
		const devSystemExtensionsLocation = URI.file(path.normalize(path.join(FileAccess.asFileUri('').fsPath, '..', '.build', 'builtInExtensions')));
		for (const extension of devSystemExtensionsList) {
			const controlState = builtinExtensionControl[extension.name] || 'marketplace';
			switch (controlState) {
				case 'disabled':
					break;
				case 'marketplace':
					devSystemExtensionsLocations.push(joinPath(devSystemExtensionsLocation, extension.name));
					break;
				default:
					devSystemExtensionsLocations.push(URI.file(controlState));
					break;
			}
		}
		const result = await Promise.all(devSystemExtensionsLocations.map(async location => this.extensionsScanner.scanExtension((await this.createExtensionScannerInput(location, false, ExtensionType.System, true, language)))));
		this.logService.trace('Scanned dev system extensions:', result.length);
		return coalesce(result);
	}