Fixes Vulkan backend.

This includes fixes for SkPicture::cullRect and SkShadowUtils updates.

* Add debug mode background

* Review notes

Update buildtools to c8db819853bcf8ce1635a8b7a395820f39b5a9fc and restrict downloaded artifacts. (#3611)

This updates our compilers, gn and ninja. Also, we now explicitly specify the buildtools artifacts we actually use instead of downloading everything from the catalog.

The build error was due to unused lambda captures being reported as warnings (which we turn into errors).

The other libraries don't repeat the "dart" name in their path inside the
package.

Mainly improvements to javadoc and error logging. A bit of refactoring.

This name is consistent with how the other `dart:` libraries are named now.

Makes it much easier to get it running under gdb.

* Update Skia to 30229ac6282981c28ced8f513c8d09684d9d0581 and fix build.
* Update licenses.

We were unnecessarily alarming users when they used the software backend. Fixes https://github.com/flutter/flutter/issues/9394

Also, adds trace events so profiling the software backend is easier.

[fuchsia] change VulkanRasterizer member ordering to avoid crashes during teardown on device shell logout (#3597)

This allows tools to locate them more easily.

Revert "Update buildtools to cdf612890f1eb65b06d5cc1df229dd44c13ee507 and restrict downloaded artifacts. (#3594)" (#3596)

This reverts commit 17da4ebc.

Update buildtools to cdf612890f1eb65b06d5cc1df229dd44c13ee507 and restrict downloaded artifacts. (#3594)

This updates our compilers, gn and ninja. Also, we now explicitly specify the buildtools artifacts we actually use instead of downloading everything from the catalog.

Keep the VulkanProcTable around long enough to use it from the destructor of the VulkanApplication

Fixes https://github.com/flutter/flutter/issues/9339

Ensure that both self.text and self.selectedTextRange are updated before
triggering textDidChange: or selectionDidChange: on the input delegate
(which then re-computes positions). This prevents inconsistencies in
selection vs text position/offset lookups triggered by didChange
notifications.

Ensure that rangeOfComposedCharacterSequenceAtIndex: is only ever called
for index values > 0 && < self.text.length. This prevents such calls for
position with index == self.text.length.

Also reduces unnecessary textDidChange/selectionDidChange notifications:
replaceRange:withText: and setMarkedText:selectedRange: are only ever
called by UIKit and methods called by UIKit (insertText,
deleteBackward), so notification is unnecessary.

* Remove the Point class.

* Add Size.contains and Rect.translate for consistency

* Make Offset and Size compare == with subclasses.

Without this we would break the _DebugSize hack.

* Fix copy/pasta

* Fix the examples in the engine repo

textContentType was added in iOS 10, and we target iOS 8. Either way the
framework doesn't (yet) include support for this; keyboard type covers
most bases.

Supports:
* autocorrect, suggestions view/selection.
* Support for Chinese, Japanese, Korean and other languages using
  multi-stage input with candidate selection.
* styling the composing (mark) region for multi-stage input
* key-repeat when backspace held down
* physical and soft keyboard support for arrow key cursor movement and
  selection.
* improved third-party keyboard support

Does not yet support:
* force-touch drag cursor positioning
* auto-positioning the candidates view for multi-stage input methods
  when used with a physical keyboard (e.g., iPad Pro) currently
  positioned at 0,0.
* voice dictation

The Google style guide specifies a limit of 100 columns.
https://google.github.io/styleguide/objcguide.xml?showone=Line_Length#Line_Length

The Chromium style guide specifies 80.
https://chromium.googlesource.com/chromium/src/+/master/styleguide/objective-c/objective-c.md

Update targets only included in host_debug_unopt.

This reverts commit f5d0004e.