31815 remove assert (cla: yes)

31819 Redo: Add buttons to gestures (a: desktop, cla: yes, framework)

31832 Allow DSS to be dragged when its children do not fill extent (cla: yes, f: scrolling, framework, waiting for tree to go green)

31835 Cherry-pick ADB CrOS fix to beta (cla: yes, tool)

31860 Fix prefer_const_constructors (cla: yes)

31862 iOS selection handles are invisible (#31332) (cla: yes)

31868 Handle notification errors (cla: yes, tool, waiting for tree to go green)

31871 Pick up v1.5.4-hotfix-2 engine for flutter 1.5.4 hotfix branch (cla: yes)