78143 Update docs to reflect Null Safety (cla: yes, d: api docs, d: examples, documentation, framework)

78757 [State Restoration] Code sample templates, migrate existing code samples (a: state restoration, cla: yes, d: api docs, f: cupertino, f: material design, framework, team)

78984 [SliverAppBar] Improve dartpad sample in documentation (cla: yes, d: api docs, d: examples, documentation, f: material design, framework, waiting for tree to go green)

t: xcode - 12 pull request(s)

75822 Avoid duplicating Pods-Runner xcconfig #includes (cla: yes, t: xcode, tool, waiting for tree to go green)

76665 Move iOS framework thinning into the tool as assemble target (cla: yes, platform-ios, t: xcode, tool)