* use HttpOverrides and dart:io HttpClient in flutter

* add missing package:http dependency

* update flutter packages and remove comment about createHttpClient from flutter_test

* move byte loading logic to common class, move string parsing logic to base class

* addAll doesn't work for a Uint8List

* use bytes.setRange

* undo addition to hello_world

* add newline to end of binding.dart

* and a newline for hello world

* refactor to function and add tests

* address comments on unknown length case

* alignment

* sort alaphabetically

* rename convertResponse to consolidateClientHttpClientResponseBytes.  Add header

* fix alignment in test

* Add a way to generically go back on both Material and Cupertino scaffold pages

* review

* lint

* review

* Fix text assert text

* a11y: remove SemanticsSortOrder; sort locally only; semanticsOwner post-test check

* update accessibility test framework

- default nextNodeId/previousNodeId to -1
- stop treating null as opt-out from value testing
- add `id`, `TestSemantics.root`, and `tags` to the suggested code in the TestSemantics failure message
- fix a small bug with raw string escaping
- update all tests accordingly

* fix sortKey doc

* prefer const over final

* Roll engine to b6df7a637498ca9beda1fa9cd7210e3202ea599f.

Changes since last roll:
```
b6df7a637 Roll dart to 290c576264faa096a0b3206c71b2435309d9f904. (#4771)
a6764dbd5 Add sources for Fuchsia target. (#4763)
2d5900615 [fuchsia] Remove unused header file. (#4769)
9717063b7 Revert "Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)" (#4768)
9a9814312 Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)
e74e8b35c [async] Update includes of async headers to new path (#4760)
e2c4b2760 Use Dart 2 camel case constants in the engine Dart libraries (#4766)
9c1e48434 Updates for Fuchsia roll. (#4765)
14c940e27 Switch from fxl::Mutex to std::mutex (#4764)
debf82c0b Roll Garnet (#4759)
5bffdefbb Use weak pointers to the accesibility bridge from objects vended to the UIKit accessibility framework. (#4761)
```

Document missing value types supported by `within`. Add missing tests. Simplify _sizeDistance implementation.

libtxt is now the default text renderer

* work around const lints

* update nonconst doc

For backward compatibility we keep supporting specifying the shape as a
combination of MaterialType and borderRadius, and we just use that as a
default when shapeBorder is null.

To cleanup the implementation if shapeBorder was not specified we just
translate the specified shape to a shapeBorder internally.
I benchmarked paint, layout and hit testing, with the specialized shape
clippers vs. the equivalent path clippers and did not see any
significant performance difference.

For testing, I extended the clippers/physicalShape matchers to match either the
specialized shape or the equivalent shape.

* Dump exceptions live when they happen when testing tests with flutter run.

* Update binding.dart

* Remaining code changes for Dart 2.0 SDK roll

...instead of the engine.

Also:

* Remove find.byIcon since it's identical to find.icon. (I sent mail to flutter-dev about this.)

* Fix IconData's operator== and hashCode, which had not been updated when we added fields.

* Add the byTooltip finder to the list of suggested finders.

* Make the suggested Key finder prettier.

* make time picker accessible

* use new CustomPaint a11y API

* flutter_localizations tests; use bigger distance delta

* fix am/pm control; selected values

* fix translations; remove @mustCallSuper in describeSemanticsConfiguration

* exclude AM/PM announcement from iOS as on iOS the label is read back automatically

This allows the scheduler library to depend on the services library
and the painting library to depend on the scheduler library without
the services library having to depend on the scheduler library.

While I was at it I also cleaned up some of the binding logic: the
licenses logic can now be overridden (and the test library does so),
and the image cache can now be overridden as well.

* Emulate text entry in FlutterDriver

* document enterText behavior

* remove the unnecessary composint TextRange

* Add a minimum distance that needs breaking on iOS each time scrolls stopped.

* Testing and tests

* tweak docs

* review

Fix the dismissible demo in the gallery (make it actuall update when you pick something from its menu; give it a better affordance for resetting once you've dismissed everything).

Improve some docs.

Fix various flinging bugs with dismissible. Add tests for those cases.

Add a feature to flutter_test to support a drag-then-fling gesture (used by the flinging tests).

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

* Revert "Revert "fix --force-upgrade script; upgrade to the latest package versions (#12700)" (#12729)"

This reverts commit 7f0d4f4c.

* keep linter pinned at 0.1.35

This reverts commit 5b357500.

* fix updrade script; upgrade to the latest package versions

* exclude special dependencies from transitive closure

* fix stack trace handling in flutter_test due to stack_trace change

* change type on _emptyStackTrace

Also:

 * Provide a better message when you lerp from infinity to finity
   constraints.

 * Make the striped marker support RTL.

 * By popular demand, dump a warning to the console the first time
   a particular Flex overflows. (Resets on hot reload.)