71689 Minor doc, style, and perf updates to Navigator/Routes (cla: yes, framework)

71707 Add stretch property to CupertinoSliverNavigationBar (cla: yes, f: cupertino, framework)

71756 Correct text selection pivot points (cla: yes, f: material design, framework, waiting for tree to go green)

71783 circleAvatar: foreground Image uses background Image as a fall-back  (cla: yes, f: material design, framework, will affect goldens)

71807 StandardMethodCodec.decodeEnvelope should allow null error message (a: null-safety, cla: yes, framework)

71830 Fix a ConstantTween’s clerical error and Add some Tween testcases. (cla: yes, framework, waiting for tree to go green)