Add `InputDecorationTheme.merge` (#129011)

fixes [[Proposal] `InputDecorationTheme.merge()`](https://github.com/flutter/flutter/issues/125471)
https://dart.googlesource.com/external/github.com/flutter/flutter/+/2e05371c88da377d72bb1de95dcd25b4cbe3fe1a
1 file changed
tree: c381bf633f71b77b7854c78e2f9b13bf570b8fac
  1. commits.json