Fixes #8200

Fixes #7166

Also cleans up from an aborted attempt at #5793

Replaces unprintable characters with alternative symbols.

If a developer has installed the Xcode command-line tools, then later
installs Xcode, they may need to run xcode-select to update the path of
the command-line tools to that of the new installation.

This reverts commit 8da4778e.

* Update hello_world to new gradle based build flow

* Update flutter_gallery to gradle based build

* Align hello_services with new gradle based build

* Update platform_services to new gradle based build

* Upgrade stock to gradle based build

The "old" APK build did not include Roboto fonts, but the new
Gradle-based build did. This is due to `flutter build flx` defaulting to
include the fonts, where the old `flutter build apk` defaulted to NOT
include them.

So let's change the Gradle build to also not include Roboto fonts.

Fixes #8149

The replacements are ListView and SingleChildScrollView, respectively.

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This mechanism has been replaced by slivers.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.