Fixes https://github.com/flutter/flutter/issues/8017

We can't read our size in the offset setter because we might be in the middle
of layout. This issue also occured with the sliver-based viewport.

* Roll engine to b64c88766dbd83bf4bab1cd1cd1757139b6cdb74.

Picks up changes to include inlined frames and line numbers in AOT stack traces.

* Adjust/weaken stack_trace_test.dart to pass in the face of async causal stack traces (Issue #8128).

Since RelativeRect's whole purpose in life is to make work with Stack
easier, it's silly that you can't directly use it with Positioned.

Also, tests for RelativeRect.
And fixes for the bugs that found...

Also use the word "dropdown" consistently over "drop down".

... and run some more tests on Windows.

This feature is wired in differently in the new scrolling world.

This mechanism has been replaced by slivers.

This is a workaround to #8141

On flutter run, we update ios/Flutter/Generated.xcconfig with various
Flutter-specific settings required by xcode_backend.sh during a build
from Xcode. These settings need to be present at the time the project is
loaded since Xcode doesn't pick up live updates to these files.

Without these settings, Xcode fails to locate xcode_backend.sh itself,
causing the build to fail until the Xcode project has been closed and
re-opened. This also prevents Xcode's project updater from 'helpfully'
suggesting to clean up and delete the Generated.xcconfig file.

* Move .iml file back to project root directory

* Update .iml location in all examples

Also, actually run the flutter_markdown tests.

Also, rename ScrollableMetrics to ScrollMetrics, which follows the
naming convention for most of the other classes (e.g., ScrollPosition,
ScrollPhysics).

Finally, fix a bug whereby SingleChildScrollView could not have a
GlobalKey, because, write test, find bug.

flutter_tools testing requires the FLUTTER_TOOLS environment variable to
be set.

* Add note about ensuring that no devices are connected

- MaterialList
 - ScrollableList
 - ScrollableLazyList
 - LazyBlock

All of these widgets can be replaced by ListView now.

The new scrolling machinery doesn't support snap scrolling.

Also fixes a bug when trying to update the offset of a viewport in a layout
builder.

Fixes #8054

For measuring the Dart thread, we care about thread duration (tdur) rather than
wall duration (dur) because we don't want to count the time when the Dart
thread is descheduled (e.g., in preference to the raster thread).

Prior to this change, these benchmarks were mostly measuring whether the OS
decided to finish the Dart thread's time slice or hand over the CPU to the
raster thread to complete the visual part of the frame. Now we actually measure
the work done on the Dart thread.

I've been using these locally, but they're likely to be useful to other people
as well.

When a ListView scrolls, it generates a LayoutChangedNotification, which was
causing Material to repaint unconditionally. That's not necessary if there are
no ink effects that need to be moved. This patch skips the repaint in that
case.

Fixes #7937

Fixes: #8087.

* move flutter user analysis options file
* add comments referencing each of the analysis options files

The old release contained some unused files that caused trouble on the Windows Chromebot.