This CL
* rolls `engine.version` to flutter/engine@b148e628 (which includes dart sdk 2.1.0-dev)
* rolls `goldens.version` to flutter/goldens@6c45fafdf (which includes updates due to skia changes in engine)
* changes `platform.dill` to `platform_strong.dill` in various places due to flutter/engine@a84b210b
* adds explicit `environment: sdk: ">=2.0.0-dev.68 < 3.0.0"` constraints to `pubspec.yaml` and `pubspec.yaml.tmpl` files (since pub defaults to `<2.0.0` if omitted)
* upgrades to newer versions of various 3rd party packages (to ensure transitive dependencies have `<3.0.0` sdk constraint)
Dart is migrating to .dart_tool/ as the location for cached artifacts
and other temporary files generated by tooling in the SDK. As part of
this, pub will be migrating from .pub to .dart_tool/pub.
In future, this path will also be used for other tooling, such as package:build.
Allows the user to specify the kind of project to create. The default is 'app'. Other choices are 'plugin' (the old '--plugin' behavior), and 'package'.
A Flutter 'package' is a Dart package that depends on Flutter, but does not contain native code.
Fixes#10377.