Currently, all our host unit-tests that have rendering concerns use the software backend because of OpenGL ES availability and stability issues on the various platforms where we run host tests. Unfortunately, entire subsystems are disabled (and not tested) when rendering with the software backend. This patch pulls in SwiftShader and via pending patches in the buildroot, configures the host unit-tests to optionally use OpenGL ES in a stable manner without relying on the OpenGL drivers being present (and functional).

I have wired up the embedder test fixture in this patch to use the SwiftShader based OpenGL ES driver. I will update the shell and runtime unittests in a subsequent patch as well. The on and offscreen surfaces are configured as 1x1 pbuffer surface because we should be able to write pixel tests using OpenGL directly wihout having to deal with surfaces.

Corects a bnuch of typeos throughout teh engien codebsae. Also makes
a couple minor Commonwealth -> US spelling adjustments for consistency
with the rest of Flutter's codebase.

Made use of `misspell` tool:
https://github.com/client9/misspell

This does not actually import the runners into the engine. It only sets up the targets so they need no modifications are necessary when the migration is done. The engine has been verified to build in both buildroots.

As long as the Mac SDK is i386 ready, engine developers should be able to use the latest version of Xcode to build all artifacts. This issue goes away when gen_snapshot migrates to x64.

This argument has been defunct for a while. It was originally added as a
means of testing the AOT machinery in debug modes.

dart_platform_sdk is not available in Android builds

Allows enabling/disabling building trained snapshots for dart2js and
dartdevc.

Reland ""Add support for trace counters with variable arguments and instrument the raster cache." (#8145)

This reverts commit bc901324 and fixes the
discovered on Windows builds.

Adds --clang-static-analysis option to GN to enable/disable compiling
with clang static analysis enabled.

This code path isn't stable yet.

Fixes flutter/flutter#27718
Reopens flutter/flutter#23838

MSVC won't compile wuff...

Fixes https://github.com/flutter/flutter/issues/23838

Background:

Some allocators (tcmalloc) don't provide alignment guarantees that
match clang's default expectations, which causes crashes when using
clang compiled binary with such allocators.

For example clang can produce movaps instructions to initialize newly
allocated object - which would crash if result of ::operator new() is
not 16 bytes aligned.

Skia privately brings in its own copy. https://skia-review.googlesource.com/c/skia/+/172147

Updates buildroot to https://github.com/flutter/buildroot/pull/180.

* Roll buildroot to be483cb1cd3a9c4313b2e534034d23a05c3d849e
* Rolls buildtools to c1408453246f0475547b6fe634c2f3dad71c6457

Adds --dynamic and --interpreter flags to
tools/gn. These flags result in engines with
properties as follows:

--dynamic:
- JIT targeting native code on Android and
  DBC on iOS

--interpreter
- Target DBC even if running on Android.

For example:

gn --android --dynamic --interpreter --runtime-mode release

Will generate an engine:
- Without Dart asserts
- Without Observatory
- With JIT compililation to DBC

into out/android_dynamic_release_dbc