32641 Updating dart.dev related links (cla: yes, d: api docs, framework)

32654 Tabs code/doc cleanup (cla: yes, f: material design, framework)

32656 Add diagnostics around needsCompositing (cla: yes)

32686 enable lint prefer_null_aware_operators (cla: yes, framework)

32702 Revert “Show/hide toolbar and handles based on device kind” (cla: yes)

32703 Add Doc Samples For CheckboxListTile, RadioListTile and SwitchListTile (cla: yes, d: api docs, f: material design, framework)

32704 Redo: Show/hide toolbar and handles based on device kind (cla: yes)

32706 Change the way macOS app names are located (cla: yes)