After this patch, Flutter apps on Fuchsia can access their asset
bundles. Unlike other platforms, the asset bundles are stored in memory
rather than in the file system.

This patch abstracts where the content of the zip file is stored.
Currently, zip files are stored in the file system, but in Fuchsia,
we're going to store them in memory (at least for the time being).
Rather than represent a zip file as a path in the file system, we
instead use an UnzipperProvider, which can create zip::UniqueUnzipper
objects on demand.

Pulls in fixes related to incremental builds of Dart sources that end up in the snapshotter.

* Remove references to unused GN files and flags.
* Fix Linux build.

The name of the generated packages file now includes the target name.

Rework the skia/BUILD.gn to be more disciplined about not leaking private config information to dependents. (#2996)

* Move all private config information from skia_config to skia_library_config. This was causing most skia specific defines to be applied to all targets that depended on Skia. These defines were only used by the Skia implementation. Makes the command line invocation for our sources smaller and less confusing.
* Removes defines not present in the current Skia sources.
* Dont add both directories containing SkUserConfig.h from ext and the Skia sources. Depending on the order in which the config directory was specified on the command line, the sources would use a different config.

In older versions of Skia, there were two files named SkFontMgr_fontconfig.cpp.
We want to use the one that contains SkFontMgr_FCI.  That file was renamed
to SkFontMgr_FontConfigInterface.cpp

This caused the assert in https://github.com/flutter/flutter/issues/5694

This code is unused.

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)