...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.

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

* Address review comments.
Test with 2 different instances of file systems.

Follow-up to #7625

Required for https://github.com/flutter/flutter/issues/138

fixes #7623

I plan to use this to implement similar logic in SliverPadding.

To make this easier to test I extended the paints matcher to accept a
function that takes a canvas. While I was at it I also made it accept
a Finder, it'll go and find the render object for you.

Also added support for paints..path and fixed some grammar in the
error messages.

Also improved the docs for debugPaint*.

This widget is a replacement for ScrollableViewport that uses the new
Scrollable2 machinery. The widget is not based on Slivers but does use the new
scroll behavior classes.