TextField FocusNode attach location change

Summary

Context

Description of change

Migration guide

Timeline

References

Summary

EditableText.focusNode is now attached to a dedicated Focus widget below
EditableText.

Context

A text input field widget (TextField, for example) typically owns a FocusNode.
When that FocusNode is the primary focus of the app, events (such as key presses) 
are sent to the BuildContext to which the FocusNode is attached.

Description of change

This change does not involve any public API changes but breaks codebases relying on 
that particular implementation detail to tell if a FocusNode is associated with a
text input field.

This change does not break any builds but can introduce runtime issues, or
cause existing tests to fail.

Migration guide