Fixes https://github.com/flutter/flutter/issues/7963

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.