This is intended to match the exit codes returned by the Dart command line tool

This patch should contain all the code we need to run Flutter on
Fuchsia's software framebuffer (without text). I haven't actually tried
running the code, so I'm sure it doesn't work yet.

Now lib/ui depends on sky/engine rather than the reverse.

* Move dart:mojo.internal to //flutter/lib/mojo

To be parallel with the other built-in libraries.

* Move DartRuntimeHooks into //flutter/lib/ui

This code manipulates natives.dart, which is part of //flutter/lib/ui.

There are now three ways to start Dart applications:

1) From a precompiled snapshot
2) From a snapshot
3) From source

When loading from source, we no longer use the complex data pipe /
dependency catcher mechanism. Instead, we just load the source
synchronously from the file system. This is appropriate because running
from source is used only during development and not during production.

The previous design was built assuming we would want to run from source
over the network, which meant we needed to be able to load source
asynchronously over a high latency channel.

Hopefully this will build this time.

This solves two problems:
* UIDartState was being deleted during destruction of DartController after the
  isolate had been shut down.  The UIDartState held persistent handles to Dart
  objects, and deleting them when the isolate no longer exists caused an
  assertion failure.
* DartStates created for secondary isolates were never being deleted

Fixes https://github.com/flutter/flutter/issues/2549

We used to share memory between Dart strings and WTF::String objects by
way of the Dart externalized strings. That used to be important when the
DOM shared many strings between C++ and Dart. However, now that we don't
retain strings in C++ much anymore, we don't need this complexity.

This patch removes DartStringCache and the integration. It also unwinds
several cases where we were converting back and forth between
WTF::String and std::string for no reason. Now we use std::string more
consistently.

For the case of ParagraphBuilder::addText, we now take a raw const
char*, which more closely matches the API the DartVM exposes. That means
we do a single copy out of the VM and into the render tree at that
point.

Moving the microtask queue into tonic solves three problems:

1) Removes three levels of indirection when invoking microtask
callbacks.
2) Removes the sky/engine/dom directory entirely.
3) Removes the last client of the (inefficient) DartValue class.

* Start capturing via <obs_host>:<obs_port>/flutter_startTracing
* Stop capturing via <obs_host>:<obs_port>/flutter_stopTracing

After this patch View is just a shim on top of Window and can be removed
easily.

Currently this code isn't completely wired up, but this patch is an incremental
step in that direction.

I left this code along in my uber patch to remove the DOM and CSS, but it's
just as dead. Also, move lone files out of dead directories into more sensible
places.

Now this code is just in dart:ui.

This adds a new placeholder dart package updater in sky/packages. This is built
into a snapshot and compiled into the Sky engine binary using the same
mechanism as the Dart isolate.

I also added a SkyHeadless class, similar to SkyView, used for running Dart
code without a view.

traces from base and the shell view dart isolate simultaneously.
Trace files contain samples from multiple sources and must be merged
separately

We no longer actually need to depend on this code. It's cleaner if we don't
list these dependencies.

TBR=eseidel@google.com

Review URL: https://codereview.chromium.org/1229113003 .

Also, remove the non-SkyView client of DartController to simplify the code
further.

R=chinmaygarde@google.com

Review URL: https://codereview.chromium.org/1210253005.

This code existed to service WebView, which no longer exists.

R=eseidel@chromium.org, eseidel@google.com

Review URL: https://codereview.chromium.org/1209033016.

This CL also removes the HTML parser and the HTML import system.

TBR=eseidel@google.com

Review URL: https://codereview.chromium.org/1215103007.

Unbreak support for snapshots now that we have our offline Dart loader. Also,
wire up support for snapshots in sky_shell on Linux.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1212623002.

This CL makes it possible to run Sky apps offline by introducing new
DartLibraryProvider subclasses, specifically one that can load directly from
the file system. A future CL will expand this functionality to work with asset
bundles as well.

Currently, the only platform that uses this functionality is Linux, which can
now load a simple Sky app without even loading a network stack. Making this
work on other platforms is work for future CLs.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1203143004.

This CL factors the network interactions out of DartLoader into a
DartLibraryProvider interface, paving the way for other library providers
(e.g., offline).

As part of this CL, I've renamed DartLoader to DartLibraryLoader and moved the
class into tonic, where it can't have any direct network dependencies.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1202283004.

This CL adds the ability to load Dart snapshot files created by sky_packager in
Sky. Using a snapshot lets us transmit all the code for an app in a single blob
and should improve startup time. Later CLs will make this codepath easier to
use and evaluate performance.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1197133004.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1153673005

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1147413003

This CL adds a global view object that can receive a Picture and be signaled to
draw. When using SkyView, this Picture shows up on screen.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1134913003

This CL makes the SkyView codepath smart enough to print "hello, world" to the
console. The code path is off by default but can be enabled by changing one
line of code.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1131673011

This CL implements SkyView::Load to start executing Sky content directly from
Dart's main(). This code isn't currently wired up to anything, so it's not yet
tested.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1130353009

After this CL, you can use <sky-element> to describe custom elements. The
current iteration is very basic and is hardcoded to "example", but its a start.

This CL renames the |init| function to |_init| to prevent importers from
calling it directly. Also, we now pass the <script> element to |_init| to give
some context.

R=ojan@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/950493003

This does several things:
1.  Teaches sky about asynchronous script execution. Previously once all imports
were loaded and the script text was available, we executed a script and assumed
it completed synchronously.  We left the parser loop to do so, but that was fine
as the next chunk from the background thread would resume the parser.  In this
change scripts now load and execute separately.  The "load" step may trigger
further dart import loads which may cause the execution to happen asynchronously
which required teaching both the DartController and the HTMLScriptRunner to
take callbacks to allow HTMLDocumentParser to know to continue parsing after
the Dart script has resolved its imports and executed.

This required re-working some of how the parser executes scripts and I
re-purposed isWaitingForScripts to include "is the parser blocked" where
as before it was limited only to "does the treebuilder have a script", even
though the imports system may have had pending scripts as well.

I made HTMLScriptRunner live only as long as the script it was executing
since it only contained per-script state at this point.

2.  Fixed an error reporting bug whereby we would not show errors when "init"
failed to execute, only "main".  This required using the dart_mirrors_api.h
which required adding an include path to the core build. :(

3.  Made it possible for a single sky file to contain multiple dart <script>
tags.  Each <script> is a separate library and executes as
soon as </script> is seen.  main or init is called for each.  This required
mangling "urls" for these script blocks since Dart unique's libraries by urls.
Before this change it may have been possible to do <import 'foo.sky'> and then
<script>import 'foo.sky'</script> and have it work!?

R=abarth@chromium.org
BUG=

Review URL: https://codereview.chromium.org/938623005

This CL adds the DartController and associated classes. These classes let you
load and execute dart code.

R=rafaelw@chromium.org
BUG=454613

Review URL: https://codereview.chromium.org/921903002