Rationalize Dart mojo and sky package structure.

NOTE: This CL appears far larger than it actually is for two reasons:

1) Many files were moved around to use the Dart package directory structure.
2) Many .dart files had to have import paths updated.

- Organize mojo/public/dart so that it uses standard Dart package layout
- Organize mojo/dart/apptest so that it uses a standard Dart package layout
- Organize sky/sdk so that it uses a standard Dart package layout
- Create a mojo/testing package (used by unittests)
- Introduce the 'dart_pkg' gn rule which populates gen/Config/dart-pkg
- All internally vended Dart packages must have a corresponding dart_pkg rule
- It is now possible to use dependency_overrides: in pubspec.yaml to mix internal and external package dependencies (enables analyzer, editor, webstorm usage for internal developers).
- Package root for dart content handler ends with "packages/"
- Imports of mojo package uris no longer need the "public/dart"
- mojo/public/tools/dart_package.py is a clone of mojo/public/tools/gn/zip.py
- Sky tests no longer run 'deploy_sdk' script.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1132063007

engine/src/flutter/BUILD.gn

@@ -9,6 +9,7 @@ group("sky") {
   testonly = true
 
   deps = [
+    "//sky/sdk",
     "//sky/engine/platform:platform_unittests",
     "//sky/engine/web:sky_unittests",
     "//sky/engine/wtf:unittests",

engine/src/flutter/assets/.gitignore

@@ -1 +0,0 @@
-material-design-icons

engine/src/flutter/assets/material-design-icons.sha1

@@ -1 +0,0 @@
-38ab35b5e5a6907852ad6e1a8e4a93da02f79591

engine/src/flutter/tests/services/iframe-embed-vmc.sky

@@ -8,9 +8,9 @@ import 'dart:sky';
 import 'dart:sky.internals' as internals;
 import 'dart:typed_data';
 
-import 'package:mojo/public/dart/application.dart';
-import 'package:mojo/public/dart/bindings.dart';
-import 'package:mojo/public/dart/core.dart';
+import 'package:mojo/application.dart';
+import 'package:mojo/bindings.dart';
+import 'package:mojo/core.dart';
 import 'package:mojom/mojo/shell.mojom.dart' as shell_mojom;
 import 'package:mojom/mojo/input_events.mojom.dart' as input_events;
 import 'package:mojom/mojo/view_manager.mojom.dart' as view_manager;

engine/src/flutter/tests/services/network.sky

@@ -1,16 +1,17 @@
 #!mojo mojo:sky_viewer
 <sky>
 <script>
-import '/sky/framework/embedder.dart';
 import 'dart:async';
 import 'dart:typed_data';
 import "dart:sky.internals" as internals;
 
-import 'package:mojo/public/dart/bindings.dart';
-import 'package:mojo/public/dart/core.dart';
+import 'package:mojo/bindings.dart';
+import 'package:mojo/core.dart';
 import 'package:mojom/mojo/network_service.mojom.dart';
 import 'package:mojom/mojo/url_loader.mojom.dart';
 
+import 'package:sky/framework/embedder.dart';
+
 Future<String> run(url) async {
   var networkService= new NetworkServiceProxy.unbound();
   embedder.connectToService("mojo:network_service", networkService);