49237 [flutter_tools][web] add favicon to template and update index.html (cla: yes, d: examples, team, team: gallery, tool)

49246 soft transition for the mustcallsuper in Element forgetChild (cla: yes, framework, waiting for tree to go green)

49247 contains (cla: yes, framework, waiting for tree to go green)

49253 Fix YamlMap cast error (cla: yes, tool)

49255 Fix template syntax error in WidgetController doc (a: tests, cla: yes, framework, waiting for tree to go green)

49256 Expose a color property to CloseButton (cla: yes, f: material design, framework, waiting for tree to go green)

49257 Fix the issue of TextSelection could misbehave inside a Nested Overlay (cla: yes, f: material design, framework, waiting for tree to go green)