Initialize ThemeData.visualDensity using ThemeData.platform instead of defaultTargetPlatform - #124357

Maybe add a note here that this looks at the true platform and not what is given from ambient ThemeData. The "see also" implies that, but it may be good to explicitly state it.

The first sentence explicitly says it is based on defaultTargetPlatform. There's also no context involved here, so it's impossible for it to depend on the ambient ThemeData, so I'm concerned that if I add a reference to something that requires a context that people will get confused as to why I'm referencing it here (and they might wonder where a context comes into it).

I added a second reference to defaultTargetPlatform, perhaps that helps make it clearer?

What I was referring to was confusion that started my whole thread. When I first saw that the buttons/ThemeData default to adaptivePlatformDensity, I thought that it would come from the platform specified by ambient ThemeData. When digging into it I realized it does not. This fix still does not make it come from an ambient theme data as we discussed because it can't. However, now ThemeData does not default to this, and it (and by proxy, buttons) will respect the density of the platform from the data.

adaptivePlatformDensity is no longer used as the default, but still exists, and still uses the defaultTargetPlatform to return a density. The doc comment above does explicitly state that, but without knowing what defaultTargetPlatform is, others could believe (like I did originally) that it respects a platform set by the theme. I think explicitly saying something like this instead would make it crystal clear:

  /// Returns a [VisualDensity] that is adaptive based on the
  /// [defaultTargetPlatform]. **This is the platform on which
  /// the framework is currently executing**.

This takes straight from the defaultTargetPlatform docs saying:
"The TargetPlatform that matches the platform on which the framework is currently executing."

Okay, I added something similar, and also a "See also" for defaultTargetPlatform.

Perfect. I'm trying to save some headache in understanding for people who think like me 😄