Declare tasks programatically rather than declaratively

This has a few benefits:

* It allows us to dynamically choose task dependencies (so for example
  the Linux standalone package can depend on the native executable
  task when running on Linux).

* Similar tasks can be defined programmatically.

* Task names are decoupled from Dart function names, which in turn
  allows us to avoid manually namespacing the package in favor of
  encouraging it to be imported with a Dart namespace.

* It works around google/grinder.dart#337 and google/grinder.dart#338.

README.md

@@ -6,3 +6,27 @@ and non-Dart users alike. It also integrates with Travis CI to make it easy to
 automatically deploy packages.
 
 [Grinder]: https://pub.dev/packages/grinder
+
+To use this package, import `package:cli_pkg/cli_pkg.dart` and call
+[`pkg.addAllTasks()`][] before calling [`grind()`][]:
+
+[`pkg.addAllTasks()`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/addAllTasks.html
+[`grind()`]: https://pub.dev/documentation/grinder/latest/grinder/grind.html
+
+```dart
+import 'package:cli_pkg/cli_pkg.dart' as pkg;
+import 'package:grinder/grinder.dart';
+
+void main(List<String> args) {
+  pkg.addAllTasks();
+  grind(args);
+}
+```
+
+The following sets of tasks are provided, each of which can also be enabled
+individually:
+
+* [Creating standalone archives for your package.](doc/standalone.md)
+* [Uploading standalone archives to GitHub releases.](doc/github.md)
+
+It's strongly recommended that this package be imported with the prefix `pkg`.

doc/github.md

@@ -0,0 +1,53 @@
+These tasks upload [standalone packages][] to GitHub releases. They're enabled
+by calling [`pkg.addGithubTasks()`][], which automatically calls
+[`pkg.addStandaloneTasks()`][]
+
+[standalone packages]: standalone.md
+[`pkg.addGithubTasks()`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/addGithubTasks.html
+[`pkg.addStandaloneTasks()`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/addStandaloneTasks.html
+
+## `pkg-github-release`
+
+Uses configuration: [`pkg.humanName`][], [`pkg.version`][],
+[`pkg.githubRepo`][], [`pkg.githubReleaseNotes`][]
+
+[`pkg.humanName`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/humanName.html
+[`pkg.version`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/version.html
+[`pkg.githubRepo`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/githubRepo.html
+[`pkg.githubReleaseNotes`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/githubReleaseNotes.html
+
+Creates a GitHub release for the current version of this package, without any
+files uploaded to it.
+
+## `pkg-github-$os`
+
+Depends on: [`pkg-standalone-$os-ia32`, `pkg-standalone-$os-x64`][]
+
+[`pkg-standalone-$os-ia32`, `pkg-standalone-$os-x64`]: standalone.md#pkg-standalone-os-arch
+
+Uses configuration: [`pkg.version`][], [`pkg.githubRepo`][], [`pkg.standaloneName`][]
+
+[`pkg.standaloneName`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/standaloneName.html
+
+Uploads 32- and 64-bit executable packages for the given operating system
+(`linux`, `windows`, or `macos`) to the GitHub release for the current version.
+
+Any OS's packages can be built and uploaded regardless of the OS running the
+task, but if the host OS matches the target OS the 64-bit executable will be
+built as a native executable, which is substantially faster.
+
+This must be invoked after [`pkg-github-release`][], but it doesn't have a
+built-in dependency so that different OSs' packages can be built in different
+build steps.
+
+[`pkg-github-release`]: #pkg-github-release
+
+## `pkg-github-all`
+
+Depends on: [`pkg-github-release`][], [`pkg-github-linux`, `pkg-github-macos`,
+`pkg-github-windows`][].
+
+[`pkg-github-linux`, `pkg-github-macos`, `pkg-github-windows`]: #pkg-github-os
+
+A utility task for creating a release and uploading packages for all operating
+systems in the same step.

doc/standalone.md

@@ -0,0 +1,70 @@
+These tasks create self-contained archives containing the Dart VM and snapshots
+of the package's executables, which can then be easily distributed. They're the
+basis for many other tasks that upload the packages to various package managers.
+They're enabled by calling [`pkg.addStandaloneTasks()`][].
+
+[`pkg.addStandaloneTasks()`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/addStandaloneTasks.html
+
+## `pkg-compile-snapshot`
+
+Uses configuration: [`pkg.entrypoints`][]
+
+[`pkg.entrypoints`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/entrypoints.html
+
+Output: `build/$entrypoint.snapshot`
+
+Compiles each executable in the package to a [kernel snapshot][snapshot].
+
+[snapshots]: https://github.com/dart-lang/sdk/wiki/Snapshots
+
+## `pkg-compile-native`
+
+Uses configuration: [`pkg.entrypoints`][], [`pkg.version`][]
+
+[`pkg.version`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/version.html
+
+Output: `build/$entrypoint.native`
+
+Compiles each executable in the package to a native code snapshot (what Dart
+calls an ["AOT Application snapshot"][snapshot]). This is unavailable on 32-bit
+host systems.
+
+Defines an environment constant named `version` set to [`pkg.version`][] that
+can be accessed from within each entrypoint via [`String.fromEnvironment()`][].
+
+[`String.fromEnvironment()`]: https://api.dartlang.org/stable/dart-core/String/String.fromEnvironment.html
+
+## `pkg-standalone-$os-$arch`
+
+Depends on: [`pkg-compile-snapshot`][] or [`pkg-compile-native`][]
+
+[`pkg-compile-snapshot`]: #pkg-compile-snapshot
+[`pkg-compile-native`]: #pkg-compile-native
+
+Uses configuration: [`pkg.version`][], [`pkg.standaloneName`][], [`pkg.entrypoints`][]
+
+[`pkg.standaloneName`]: https://pub.dev/documentation/dart_cli_pkg/latest/cli_pkg/standaloneName.html
+
+Output: `build/$standaloneName-$version-$os-$arch.(tar.gz|zip)`
+
+Creates an archive that contains all this package's entrypoints along with the
+Dart VM for the given operating system and architecture, with top-level scripts
+that can be used to invoke them.
+
+Any OS's packages can be built regardless of the OS running the task, but if the
+host OS matches the target OS *and* the architecture is 64-bit, executables will
+be built as native (["AOT"][snapshot]) executables, which are substantially
+faster and smaller than the kernel snapshots that are generated otherwise.
+
+This produces a ZIP file in Windows, and a gzipped TAR file on Linux and Mac OS.
+
+## `pkg-standalone-all`
+
+Depends on: [`pkg-standalone-linux-ia32`, `pkg-standalone-linux-x64`,
+`pkg-standalone-macos-ia32`, `pkg-standalone-macos-x64`,
+`pkg-standalone-windows-ia32`, `pkg-standalone-windows-x64`][]
+
+[`pkg-standalone-linux-ia32`, `pkg-standalone-linux-x64`, `pkg-standalone-macos-ia32`, `pkg-standalone-macos-x64`, `pkg-standalone-windows-ia32`, `pkg-standalone-windows-x64`]: #pkg-standalone-os-arch
+
+A utility task for creating a packages for all operating systems in the same
+step.

lib/cli_pkg.dart

@@ -12,57 +12,15 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-import 'package:grinder/grinder.dart';
-
-import 'github.dart' as github;
-import 'standalone.dart' as standalone;
-
-export 'src/info.dart' hide pubspec;
-
-// Manually export tasks to work around google/grinder.dart#337
-
-@Task('Build Dart script snapshot(s).')
-void pkgCompileSnapshot() => standalone.pkgCompileSnapshot();
-
-@Task('Build Dart native executable(s).')
-void pkgCompileNative() => standalone.pkgCompileNative();
-
-@Depends(pkgCompileSnapshot)
-@Task('Build a standalone 32-bit package for Linux.')
-Future<void> pkgStandaloneLinuxIa32() => standalone.pkgStandaloneLinuxIa32();
-
-@Task('Build a standalone 64-bit package for Linux.')
-Future<void> pkgStandaloneLinuxX64() => standalone.pkgStandaloneLinuxX64();
-
-@Depends(pkgCompileSnapshot)
-@Task('Build a standalone 32-bit package for Mac OS.')
-Future<void> pkgStandaloneMacOsIa32() => standalone.pkgStandaloneMacOsIa32();
-
-@Task('Build a standalone 64-bit package for Mac OS.')
-Future<void> pkgStandaloneMacOsX64() => standalone.pkgStandaloneMacOsX64();
-
-@Depends(pkgCompileSnapshot)
-@Task('Build a standalone 32-bit package for Windows.')
-Future<void> pkgStandaloneWindowsIa32() =>
-    standalone.pkgStandaloneWindowsIa32();
-
-@Task('Build a standalone 64-bit package for Windows.')
-Future<void> pkgStandaloneWindowsX64() => standalone.pkgStandaloneWindowsX64();
-
-@Task('Build all standalone packages.')
-Future<void> pkgStandaloneAll() => standalone.pkgStandaloneAll();
-
-@Task('Create a GitHub release, without executables.')
-Future<void> pkgGithubRelease() => github.pkgGithubRelease();
-
-@Task('Release Linux executables to GitHub.')
-Future<void> pkgGithubLinux() => github.pkgGithubLinux();
-
-@Task('Release Mac OS executables to GitHub.')
-Future<void> pkgGithubMacOs() => pkgGithubMacOs();
-
-@Task('Release Windows executables to GitHub.')
-Future<void> pkgGithubWindows() => pkgGithubWindows();
-
-@Task('Create a GitHub release with all executables.')
-Future<void> pkgGithubAll() => pkgGithubAll();
+import 'src/github.dart';
+import 'src/standalone.dart';
+
+export 'src/info.dart';
+export 'src/github.dart';
+export 'src/standalone.dart';
+
+/// Enables all tasks from the `cli_pkg` package.
+void addAllTasks() {
+  addGithubTasks();
+  addStandaloneTasks();
+}