Process.run does not respect environment on Windows

[ghost]

I'm observing some inconsistencies in behaviour when using Process.run(..) on Windows as follows:

When I run the below I get "ProcessException: The system cannot find the file specified.":

  Map<String, String> env = {'PATH': 'C:\\src\\'};
  result = await Process.run('hello.bat', [], environment: env);
  print(result.stdout);

This is unexpected as I can otherwise run the executable fine when I either run directly, without setting PATH:

  ProcessResult result;
  result = await Process.run('C:\\src\\hello.bat', []);
  print(result.stdout);

Or run it with PATH but with runInShell:

  Map<String, String> env = {'PATH': 'C:\\src\\'};
  result = await Process.run('hello.bat', [], environment: env, runInShell: true);
  print(result.stdout);

This indicates to me that it is able to run the process fine, and even pass the environment along in some cases - just not in the non-runInShell case, which seems like a bug.

[ghost]

FWIW, the failing example above appears to work fine one other platforms.

[brianquinlan]

The CreateProcess documentation says:

If the file name does not contain a directory path, the system searches for the executable file in the following sequence:

1. The directory from which the application loaded.
2. The current directory for the parent process.
3. The 32-bit Windows system directory. Use the [GetSystemDirectory](https://learn.microsoft.com/en-us/windows/desktop/api/sysinfoapi/nf-sysinfoapi-getsystemdirectorya) function to get the path of this directory.
4. The 16-bit Windows system directory. There is no function that obtains the path of this directory, but it is searched. The name of this directory is System.
5. The Windows directory. Use the [GetWindowsDirectory](https://learn.microsoft.com/en-us/windows/desktop/api/sysinfoapi/nf-sysinfoapi-getwindowsdirectorya) function to get the path of this directory.
6. The directories that are listed in the PATH environment variable. Note that this function does not search the per-application path specified by the App Paths registry key. To include this per-application path in the search sequence, use the [ShellExecute](https://learn.microsoft.com/en-us/previous-versions/windows/desktop/axe/shellexecute) function.

My interpretation is that PATH refers to the PATH environment variable of the parent process.

Python uses the same approach that we do and they have a big warning block of documentation:

We should probably do the same i.e. suggest using a full path when runInShell=false.

[brianquinlan]

I improved the documentation.