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

This pulled a refactoring of how we keep track of the primary threads.

All that's left is two task runners, which should be statics somewhere.

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.

We now just call stat directly to test whether a file exists.

This required us to fix a backwards dependency from flutter/lib/ui to
sky/shell.

We can use the one from //lib/tonic instead.

This removes most (all?) of our dependency on base::RefCountedThreadSafe.

* Adopt //lib/tonic/logging

* Adopt //lib/tonic/scopes

* Adopt //lib/tonic/parsers

Also, switch over to //lib/tonic's typed data classes.

Avoid re-entering the message loop while reloading. Fixes on stack reloading in Flutter embedder. (#2775)

This should make almost every Canvas call significantly faster.

Now that tonic doesn't depend on anything in //sky/engine anymore, we
can move the code to a location where its dependencies are clearer.

This patch paves the way to untangling a large amount of code from the engine.

This reverts commit c0a61a03.

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.

Instead of using a C++ object as a peer to the Dart wrapper, these wrapper hold
arbitrary data in the internal field of the Dart wrapper.

Instead, use C++ template to generate the callback trampolines and the
registration block.

This patch changes how we lookup native functions from Dart to be more
extensible. This patch paves the way for removing the dart:ui_internals
library.

The NewTextPainter is still disabled by default. A future patch will flip the
flag to enable it.

This patch uses a new approach to writing bindings by encoding data into array
buffers. This approach is more efficient than the existing IDL based approach.
If this works out well, we should convert our other performance-sensitive
interfaces to this approach in future patches.

* Moves from vector_math to vector_math_64
* Adds support for Float64List in Dart bindings

This updates to mojo commit d259eb58aa59 and limits the roll script to
only pull in the parts of //mojo that are currently being used. More
stuff will be dropped in the future.

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.

The array should be in column-major format, in the format used by vector_math.dart.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1155193004

BUG=
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1159443002

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1160763004

This patch adds utility functions which provide a single choke point for calling into sky application code. For now this is only serving the purpose of having a common trace event for calls into app code.

R=abarth@chromium.org, abarth
BUG=

Review URL: https://codereview.chromium.org/941153003

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/948943002