`dart migrate` should depend on `dart pub`, not `pub.bat`

[mit-mit]

Currently dart migrate tries to invoke pub.bat:

C:\Users\mit\tmp\foobar>dart migrate
CreateProcessW failed 2
ProcessException: The system cannot find the file specified.

  Command: "C:\Program Files\Dart\dart-sdk\bin\pub.bat" outdated --mode=null-safety --json

Also, it refers to pub.bat in it's user facing output:
https://github.com/dart-lang/sdk/blob/master/pkg/nnbd_migration/lib/migration_cli.dart#L159

This is not a good idead for Flutter users who are using the embedded Dart SDK. They will have the flutter and dart commands in path, but no pub command. It would be better to depend on, and direct users towards, using the dart pub command, which they will have available.

[srawlins]

Is the dart command on windows something like dart.exe?

[mit-mit]

No, they are bat scripts:
https://github.com/flutter/flutter/tree/master/bin

However, on Windows .exe and .bat extensions are recognized and expanded, so you can just type dart (for dart.bat) or notepad (for notepad.exe).

[mit-mit]

For technical details, see "PATHEXT" under Remarks in this article:
https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/start