src/vs/workbench/api/common/extHostLanguageFeatures.ts/TypeHierarchyAdapter/provideSubtypes
class TypeHierarchyAdapter:	async provideSubtypes(sessionId: string, itemId: string, token: CancellationToken): Promise<extHostProtocol.ITypeHierarchyItemDto[] | undefined> {
		const item = this._itemFromCache(sessionId, itemId);
		if (!item) {
			throw new Error('missing type hierarchy item');
		}
		const subtypes = await this._provider.provideTypeHierarchySubtypes(item, token);
		if (!subtypes) {
			return undefined;
		}
		return subtypes.map(subtype => {
			return this._cacheAndConvertItem(sessionId, subtype);
		});
	}