Currently, MaterialApp inserts a `DefaultTextStyle` in its hierarchy
with a style of `_errorTextStyle`. This means that any material app
with a `Text` widget that:
(a) doesn't have another `DefaultTextStyle` widget in its ancestry
    between it and the `MaterialApp`, and
(b) specifies an explicit style with `inherit: true`
... is susceptible to having its styles inherit from the app-level
error styles.  In this case, it's forced to insert a `DefaultTextStyle`
as a *child* of the `MaterialApp`, which is a little counter-intuitive.

This is exacerbated by the change in #12249, which changed the default
platform text themes in `typography.dart` to `inherit: true`. While this
change was correct, it makes apps more susceptible to the behavior above.
Consider the case of an app using a `Text` widget and explicitly setting
a `style` on that widget of a platform default style. In that case, they're
*explicitly* saying "I want this text to have style X" (e.g.
`Typography.white.body1`), and because the default platform text styles
don't specify decorations, those get inherited from the top-level error
text styles.

This change adds the ability to give `MaterialApp` a `defaultTextStyle`,
and only to use `_errorTextStyles` if such a default is unspecified.