src/vs/editor/browser/config/migrateOptions.ts/EditorSettingMigration/_read
class EditorSettingMigration:	private static _read(source: any, key: string): any {
		if (typeof source === 'undefined') {
			return undefined;
		}

		const firstDotIndex = key.indexOf('.');
		if (firstDotIndex >= 0) {
			const firstSegment = key.substring(0, firstDotIndex);
			return this._read(source[firstSegment], key.substring(firstDotIndex + 1));
		}
		return source[key];
	}