61401 Make scrollbar thickness and radius customizable (cla: yes, f: cupertino, f: material design, framework)

61406 Update the golden image comparator to handle read-only image ByteData views (a: tests, cla: yes, framework, waiting for tree to go green)

61413 [flutter_tools] update fastReassemble method for single widget reloads (cla: yes, framework, tool)

61424 Update documentation (cla: yes, f: material design, framework)

61425 Default textBaseline to alphabetic in Flex (cla: yes, framework, waiting for tree to go green)

61439 Add per thumb Range Slider semantics (cla: yes, f: material design, framework, waiting for tree to go green)

61455 Improve error reporting for RenderObject visitChildren errors (cla: yes, framework)