Fixes #6963

This patch replaces uses of Flexible with Expanded where we're using
FlexFit.tight. We still need to think of a better name for the
FlexFit.loose variant.

Also, improve the docs for Row, Column, Flex, and RenderFlex to be more
problem-oriented and to give a complete account of the layout algorithn.

Fixes #6960
Fixes #5169

* Add delay between writing file and modifying it

* Add type argument

- [x] Make iOS and Android application startup paths consisitent.
- [x] Remove the now unncessary Android specific special cases from the runners.

This allows us to run the analyzer CLI on packages and correctly find dart:ui.

- [x] Stop unnecessarily building the flx a second time on startup.
- [x] Stop unncessarily copying the flx to the device a second time on startup.
- [x] Remove `startBundle` and move logic into `startApp`.

We're getting rid of the http package from third_party/dart-pkg/

(and minor doc changes)

This will in the future allow Scrollable to not track the Simulation itself.

* Added comment for INTERNET permission in Android manifest

* Added more details to the permission comment.

* Allow for application-specific log readers.

When running with prebuilt application binaries, those applications
aren't guaranteed to be named "Runner" (as it is when we build
the app locally in Flutter tools)

Fixes the `package:drudge` spam.

See: #6840

...at least not in the traditional markdown style.
So let's try HTML instead.

* Bump Dart SDK to `1.21.0-dev.6.0`.

* Bump `json_schema` version.

`[IconButton]` is referenced as singular everywhere else, so the _most correct_ version would be singular (`[IconButton]`) with the `s` outside of the brackets.

More idiomatic use of constraints in performResize.

Trivial fixes to comments.

Make ProxyBox not use BoxParentData since it ignores the field.

Make applyPaintTransform more helpful if you use a different ParentData
subclass than RenderBox expects.

Make debugAssertIsValid actually fulfill its contract in RenderObject as
documented.

Add a childBefore for symmetry (we already had childAfter).

Fix the way we dump the child list when there's no children in a
multichild render object.

More asserts in the rendering test library.

...so that when you detach, you still have the slot, the parentData, etc.