fixes https://github.com/flutter/flutter/issues/7654

Combines many sliver features at once.

This structure makes it easier to customize the viewport used in a Scrollable.
Now Scrollable2 doesn't commit to using slivers.

Responding to https://github.com/flutter/flutter/pull/7640#pullrequestreview-18563416

Currently, this only calls out to the PowerShell Script, which is kind of slow. In the future, we will transfer more logic from PowerShell to cmd for faster starup time.

This remove a very brittle aspect of flutter drive, whereby it would
assume a known port instead of explicitly finding out what it was.

Fixes #7692 and hopefully fixes the devicelab tests.

Move the drive/run common stuff to RunCommandBase, rather than the
superclass which applies to everything.

Implement --no-resident (fixes
https://github.com/flutter/flutter/issues/5448). This is particularly
useful when running flutter from scripts.

Rename RunAndStayResident to ColdRunner.

Uses SliverPadding to implementing paddding in ScrollView. Also, deploy
ScrollView in more places now that it implements padding.

Finally, remove loader_app.dart because it is not referenced.

We now report an accurate estimate when the lastIndex is the final index
in the list.

Also, we now project forward from the lastIndex using the extent of the
reified children instead of trying to project forward from index 0.

This new algorithm leads to estimation error as you reach the end of a
list, avoiding some ballistic overscrolls.

...for the port arguments

Rumours have it that this is faster now.

Fixes https://github.com/flutter/flutter/issues/5154

Turns out MacOS has a really low open files limit and so if you try to
open EVERY FILE AT THE SAME TIME it falls over.

This just opens the files one at a time, the way we used to back in
the old days.

Almost all real-world apps will want a custom app delegate, and 'flutter
create' code-gens one by default. This brings the samples in line with
our templates and the most common use case.

When we start a ballisitic scroll activity to correct an out-of-range scroll
offset, make sure we start with a velocity that is headed in the right
direction.

...and add --diagnostic-port.

...and document port 0.

* Add a check in case the flutter directory in .packages no longer exists. Clean up and prompt user

* Update documentation to use flutter packages get for end-users instead of flutter update-packages.

* Merge missing sdk error with the multiple sdk error. They're really the same thing.

* Use flutterPath in both checks.

* Change file_system’s copy folder to copy director which takes into account the file system

* Test support files

* Add test and split into 2 messages again.

* Move tests to run in memory file system's copy. Tested with dev/bots/test.sh

Fixes https://github.com/flutter/flutter/issues/7661

This yields a cleaner separation between common testing code
and actual tests (a distinction which is important in Google's
internal build system).

We converted all code to use method generics in #7235, but
we shouldn't have touched the `http` folder since this code
is pulled from upstream.

The zip files downloaded from cloud storage are not clearly separated
into engine artifacts and tools anyways.

Also, because of this unclear separation, we were downloading
`linux-x64/artifacts.zip` twice before.

This patch deploys ScrollView in a number of demos and manual tests.

* Tweak the contributor guide with info on working with the flutter tool

* Slight phrasing

* Review edits on doc phrasing

* Review notes

* Small review tweak

When you put a box in a Viewport2, it might paint outside its allocated bounds.
This patch teaches Viewport2 to clip when that happens.

This patch introduces ScrollView, which is a convenience widget for using a
SliverBlock. This patch also switches a number of tests from Block to
ScrollView. Once we support more features of block (e.g., padding and
shrinkwrapping), we'll be able to move over more clients.