CupertinoSliverNavigationBar.search has the core search functionality, but needs some fidelity improvements to match native iOS.

• Icon alignments
• Search field height (35.0 -> 36.0)
• Animation for the search transition
• Placeholder text colors (see Improve fidelity of CupertinoSliverNavigationBar.search and CupertinoSearchTextField #163020 (comment))
• Overall alignment and position (see Improve fidelity of CupertinoSliverNavigationBar.search and CupertinoSearchTextField #163020 (comment))