Replicates iOS behavior of placing trace files in the documents directory.

Respect the —enable-checked-mode flag when not running precompiled code instead of defaulting it on. (#2986)

* Add trace event slices for the duration in which the engine produces a pipeline item.

* Trace the entire lifetime of an item in the pipeline.

When rasterizing to a recorder backed canvas for SKP tracing, don’t attempt to update the raster cache. (#2983)

We could get into a situation where the second time we rasterize a picture for an SKP, the raster cache detects that the same picture is being repeated in subsequent onscreen frames and then proceeds to rasterize the contents to an offscreen texture. This not only causes an unnecessary cache entry, but also shows the cached image in the final SKP trace (which is not what was originally displayed on screen).

Per recommendation from the Skia team. Benefits from optimizations applied in Skia for Chrome.

The rasterizer may have been deleted on the GPU thread before Animator::Render
runs on the UI thread

Add a temporary engine systrace logger till the Dart timeline learns how to log to systrace. (#2978)

Noticed this while looking at https://github.com/flutter/flutter/issues/5594

runtime/BUILD.gn and ..core/BUILD.gn use flutter_runtime_mode which
is defined in common.gni. If the build comes through common/BUILD.gn
this is imported (implicity) but builds that depend on specific
files, like the Fuchsia build, won't have this imported.  This
imports the gni from the build files that use it instead of relying
on it already being there.

Don’t depend on tonic/debugger in release since the Dart debugger is unavailable in product mode. (#2972)

Add “—endless-trace-buffer” to switch to an endless buffer from a ring buffer while tracing. (#2966)

Make the URL loader more resilient to dropping the start callback on the correct thread in case of early connection termination. (#2964)

Since this functionality will be removed soon, this patch removes
the logging in order not to hinder the iOS development experience.

Pipelines vend single use continuations that callers can complete at any time to signal readiness of a pipeline resource. (#2957)

If the vsync provider is being replaced, make sure the animator expects no more signals from the old registration. (#2954)

Make the animator host an implicit fallback VSync provider in case platforms cant provide their own. (#2953)

This reverts commit 26f89708.