61570 migrate scheduler to nullsafety (cla: yes, framework, waiting for tree to go green)

61572 Remove unused parameter from private method in widget_inspector (cla: yes, framework, waiting for tree to go green)

61576 Corrected splash color for TextButton, ElevatedButton, OutlinedButton (cla: yes, f: material design, framework, waiting for tree to go green)

61578 Updated obsolete button references in Scaffold, IconButton (cla: yes, f: material design, framework)

61579 Remove unused FlutterErrorDetails subclasses (a: debugging, a: error message, a: quality, a: tests, cla: yes, framework, waiting for tree to go green)

61581 Give better error message for describeEnum (a: error message, cla: yes, f: inspector, framework, waiting for tree to go green)