Looks pretty good. Two issues:

1. Our style guide says to use { / } for functions that are more than one textual line.

2. We need a test to make sure we don't regress. The test can be pretty simple. If you look in the packages/flutter/test/widgets directory, you'll see lots of examples of tests. Basically, I would use tester.pumpWidget to build a DefaultTextStyle with a pre-built Text child (i.e., one that you keep in a local variable outside the pumpWidget call) and then use find.byType(RichText) to see what values actually get used in the underlying RichText widget. Then, you can use tester.pumpWidget to make a new DefaultTextStyle with the exact same prebuilt Text child, and then again verify that one of the properties on the RichText that we failed to update before now gets updated.

Updated, fixed style and added test

LGTM

Travis would like you use one more const constructor:

[lint] Prefer const with constant constructors. (packages/flutter/test/widgets/default_text_style_test.dart, line 26, col 29)

Thanks so much! Once the bots go green, I'll merge your patch.