How do I remove content padding from TextField?

Update (August 2022): still the same as of flutter 3.0.5
Update (April 2021): still work in flutter 2.0.4

As of flutter 1.17.5 (and still the same in 2.X) to completely remove or manipulate the padding manually, first you must set isDense: true and then you can adjust the contentPadding as you wanted or apply padding on the parent widget instead.

// using theme
ThemeData(
  inputDecorationTheme: InputDecorationTheme(
     isDense: true,// this will remove the default content padding
     // now you can customize it here or add padding widget
     contentPadding: EdgeInsets.symmetric(horizontal: 0, vertical: 0),
     ...
  ),
)

// per widget
TextField(
   decoration: InputDecoration(
     isDense: true,
     contentPadding: EdgeInsets.symmetric(horizontal: 0, vertical: 0),
     ...
   ),
)

As mentioned in the comment by Ataberk you can also use contentPadding: EdgeInsets.zero

TextField(
   decoration: InputDecoration(
     isDense: true,
     contentPadding: EdgeInsets.zero,
     ...
   ),
)