Required to support Flutter SDK paths with spaces.

Required to handle Flutter SDK (and other) paths that include spaces.

Also includes general cleanup:
* Declare explicitly that we use /bin/bash, since we rely on its
  features.
* Add -- where possible, to avoid interpreting files starting in - as
  options.
* Suppress output of pushd/popd.
* Avoid stringifying arrays.

We call the builder in slightly different ways, but the new results
seem reasonable.

* Support changing the dismissable threshold for any direction for dismissable widget.

* Fixing review comments.

All the clients have migrated to GridView.  Also, remove RenderGrid,
which isn't needed by GridView.

* return 1 when checks in `flutter.bat` fail
* don't assume file extension of git (depending on the system it can be `git.exe` or `git.bat`)

* Sync CONTRIBUTING.md text with engine.git

Turns out we had very similar text in engine.git.  This patch makes the text the same by adopting the text in engine.git.

* Update CONTRIBUTING.md

We weren't triggering a relayout when the number of children changed.

Fixes #8001

* Fix potential NPE in iOS doctor check

In case Xcode is not installed, the xcode-select path may be null.

* fixup! Fix potential NPE in iOS doctor check

If the developer has only installed the Xcode command-line tools,
xcode-select and some other tools may be present, but xcodebuild will be
missing. In this case, output a slightly improved message indicating
that the installation is incomplete rather than missing.

* Add indent option to logger and indent non-flutter nested output

* Add a missed override

* Formatting

Move the back button and drawer opening logic into the app bar.

Move the tap-status-bar-to-scroll-to-top logic to using
ScrollControllers. Provide a PrimaryScrollController and a `primary`
flag on scroll views.

Make it possible to track when a route becomes or stops being poppable.

* Allow for customization of hintText in InputField

* Adding tests.

* disables all `flutter test` and `flutter drive` tests on Windows as those two commands are not fully implemented on Windows yet
* fixes other failures on Windows

All clients have moved to PageView.

This manual test has used the PageView widget for a while already.

...for the case where there's no right or bottom padding.

Tabs are now fully driven by slivers.

Turns out when I added tests, I broke the actual underlying functionality.

This reverts commit ea21d0c5 which
didn't resolve the pagination control overflow issue. Commit
7425940d replaces this as an alternate
workaround. Once flutter/flutter#7980 is resolved, we should revert
7425940d and scroll instead.

Once engine commit d49a6b110bced49fa318b0dd45bf409218df9ee5 rolls into
Flutter rolls in, we get correct font metrics for the Ahem font, used in
headless sky_shell test runs. Ahem has much wider glyphs than the system
font, which causes the PaginatedDataTable bottom bar to be much wider
and overflow the previous and next controls offscreen.

This commit reduces the number of rows such that the prev/next controls
are still just barely onscreen.

Fixes #7119

The Ahem font has a much wider average text width than most fonts.

Now that ensureVisible works, we can use ListView instead of Block in
this demo.