testing/dart: add version file

seems to be needed by some tools, e.g. dart analyzer:
https://github.com/dart-lang/sdk/blob/2.18.7/pkg/analyzer/lib/src/dart/sdk/sdk.dart#L423

[01:56:58] [Analyzer] [Error] Unhandled exception:
FileSystemException(path=/usr/lib/dart/version; message=Cannot open file)
#0      _PhysicalFile.readAsStringSync (package:analyzer/file_system/physical_file_system.dart:158:7)
#1      FolderBasedDartSdk.languageVersion (package:analyzer/src/dart/sdk/sdk.dart:427:12)
#2      Driver.start (package:analysis_server/src/server/driver.dart:295:18)
#3      main (file:///home/patrycja/aports/testing/dart/src/dart-sdk-2.19.0/pkg/analysis_server/bin/server.dart:10:11)
#4      _delayEn…