Depends on https://github.com/flutter/engine/pull/4483

* Set _imageInfo to null before adding listener to new ImageStream

Fixes an edge case where switching the ImageStream to an already completed ImageStream caused the Image widget to show nothing. ImageStreamCompleter calls the listener during addListener if their image is already loaded.

* Fix whitespace

* Add test to verify correct behavior

* Added test from mravn-google's PR

This provides a way to draw colors, images, and general decorations on Material widgets, without interfering with InkWells that are further descendants of the widget.

This thus provides a cleaner way to solve the issue of FlatButtons and InkWells not working when placed over Image widgets than the old hack of introducing a transparency Material.

Fixes #3782.

Also, some fixes to documentation, and remove a redundant property on the Image widget.

This allows the scheduler library to depend on the services library
and the painting library to depend on the scheduler library without
the services library having to depend on the scheduler library.

While I was at it I also cleaned up some of the binding logic: the
licenses logic can now be overridden (and the test library does so),
and the image cache can now be overridden as well.

* feat: add optional headers params to network image

* fix: addressed comments

* feat: add test

* style: format code

This makes sure that we don't decode frames when we don't need to run
the animation.

For example, prior to this CL: an animated image in a page that is not
currently at the top of the page stack (it is covered by another page),
will keep the the image stream listener, and thus frames will still be
decoded.

* Add caching detail in doc

* review

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

Add Diagnosticable base class and documentation

* FadeInImage: shows a placeholder while loading then fades in

* fix dartdoc quotes

* license headers; imports

* use ImageProvider; docs; constructors

* _resolveImage when placeholder changes

* address comments

* docs re ImageProvider changes; unsubscribe from placeholder

* rebase

* address comments

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

* Use defaultTargetPlatform instead of Platform.operatingSystem for ImageProvider.
Add convenience helper to make it easier to display a short string for
an enums value.

* Add comment linking to issue.

Applies comments I missed from https://github.com/flutter/flutter/pull/10904.

* more widget const constructors

* prefer const constructors

* address review comments

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

Fixes #7503
Fixes #8870

This machinery is useful for arbitrary boxes (e.g., with FittedBox).

Fixes #6463

The new name matches the style guide. (The old name was just old and predated
the style guide.)

Fixes #8000

This widget lets you apply an ImageFit to a child widget.

Fixes #5830

This allows us to simplify the logic around inherited widgets e.g. in
the Image widget.

I did a pass through some of the code cleaning minor things up.