Inconsistent `statSync().mode` Behavior: Fails to Differentiate Between Non-Existent and Permission-Denied Files

[stephane-archer]

Currently, the statSync().mode API in Dart has inconsistent behavior when dealing with non-existent files and files with restricted permissions:

touch MyFile
chmod 000 ./MyFile

var mode = File("MyFile").statSync().mode; // Returns 0 as expected (no permissions). 

var mode = File("notExistingFile").statSync().mode;  
// Also returns 0, which is misleading because the file does not exist.
// Ideally, it should throw an exception to differentiate this case.  

Expected Behavior:
An exception should be thrown for non-existent files to distinguish them from files with restricted permissions.

Actual Behavior:
Both scenarios return 0 from statSync().mode, which can lead to incorrect assumptions in applications.

[dart-github-bot]

Summary: statSync().mode returns 0 for both non-existent and permission-denied files. This inconsistency needs a fix to throw an exception for non-existent files.

[a-siva]

@brianquinlan can you take a look at this and decide if it makes sense to behave differently for the two cases.