Use case:

1. Create a shared constant app-wide theme, like so:

final appTheme = new ThemeData(
  // Define whatever styles you want, but leave the
  // text themes unspecified (and thus default).
  // In so doing, they'll get the default typography
  // values (e.g. blackMountainView), which as of
  // #12249, specify inherit:true
);

2. Create a Text widget somewhere in your app, with styles like so:

  new Text(
    'sample text',
    style: appTheme.primaryTextTheme.button,
  ),

3. The styling of the text widget will be appTheme.primaryTextTheme.button, merged with whatever DefaultTextStyle happens to live immediately above the widget in the tree. This is unexpected, since the user doesn't expect the default primaryTextTheme.button style to leave some properties ambiguously defined.

Potential solutions:

1. Do nothing, and call this working as intended. If the user doesn't want to inherit properties this way, they can do the following:

  new DefaultTextStyle(
    style: appTheme.primaryTextTheme.button,
    child: new Text(
      'sample text',
    ),
  ),

2. Add the ability to clone a TextStyle but change the inherit property, yielding user code like this:

  new Text(
    'sample text',
    style: appTheme.primaryTextTheme.button.copyWith(new TextStyle(inherit: false)),
  ),

3. ???