update_homebrew🎯 also make a dart@2, for Dart 2 migration

This PR modifies update_homebrew to also generate a dart@2 versioned formula. The new file is dropped out right next to the old `dart.rb` file.

The dart@2 formula is a temporary measure to aid application developers
migrating from Dart 1 to Dart 2.

Supersedes https://github.com/dart-lang/homebrew-dart/pull/52.

This looks right by inspection, but I haven't been able to test it yet: my local `dart` environment is all messed up, and I am a Dart newbie. Still, I'm putting this out here as a PR so the discussion from https://github.com/dart-lang/homebrew-dart/pull/52 can proceed.

Closes #33318
https://github.com/dart-lang/sdk/pull/33318

GitOrigin-RevId: 5230782aabf4f45199cff9b1fb9c5304e5dcb21c
Change-Id: I78cf5b066d85ac8b0ef51ed44e6eb1a15192b23c
Reviewed-on: https://dart-review.googlesource.com/58120
Commit-Queue: William Hesse <whesse@google.com>
Reviewed-by: William Hesse <whesse@google.com>

tools/apps/update_homebrew/lib/update_homebrew.dart

@@ -23,6 +23,7 @@ const dartiumFile = 'dartium/dartium-macos-x64-release.zip';
 const contentShellFile = 'dartium/content_shell-macos-x64-release.zip';
 
 const dartRbFileName = 'dart.rb';
+const dart2RbFileName = 'dart@2.rb';
 
 Future<String> getHash256(
     String channel, String revision, String download) async {
@@ -107,6 +108,9 @@ Future writeHomebrewInfo(
   await new File(p.join(repository, dartRbFileName)).writeAsString(
       createDartFormula(revisions, hashes, devVersion, stableVersion),
       flush: true);
+  await new File(p.join(repository, dart2RbFileName)).writeAsString(
+      createDart2Formula(revisions, hashes, devVersion, stableVersion),
+      flush: true);
 }
 
 String createDartFormula(
@@ -200,6 +204,61 @@ class Dart < Formula
 end
 ''';
 
+/// TODO: Remove this formula after Dart 2 is released as stable.
+/// This is a transitional mechanism to support developers migrating
+/// from Dart 1 to Dart 2 while it's in pre-release.
+String createDart2Formula(
+        Map revisions, Map hashes, String devVersion, String stableVersion) =>
+    '''
+class DartAT2 < Formula
+  desc "The Dart 2 SDK"
+  homepage "https://www.dartlang.org/"
+  version "$devVersion"
+
+  keg_only :versioned_formula
+
+  if MacOS.prefer_64_bit?
+    url "$urlBase/dev/release/${revisions['dev']}/$x64File"
+    sha256 "${hashes['dev'][x64File]}"
+  else
+    url "$urlBase/dev/release/${revisions['dev']}/$ia32File"
+    sha256 "${hashes['dev'][ia32File]}"
+  end
+
+  def install
+    libexec.install Dir["*"]
+    bin.install_symlink "#{libexec}/bin/dart"
+    bin.write_exec_script Dir["#{libexec}/bin/{pub,dart?*}"]
+  end
+
+  def shim_script(target)
+    <<~EOS
+      #!/usr/bin/env bash
+      exec "#{prefix}/#{target}" "$@"
+    EOS
+  end
+
+  def caveats; <<~EOS
+    Note that this is a prerelease version of Dart.
+
+    Please note the path to the Dart SDK:
+      #{opt_libexec}
+    EOS
+  end
+
+  test do
+    (testpath/"sample.dart").write <<~EOS
+      void main() {
+        print(r"test message");
+      }
+    EOS
+
+    assert_equal "test message\n", shell_output("#{bin}/dart sample.dart")
+  end
+end
+''';
+
+
 Future runGit(List<String> args, String repository,
     Map<String, String> gitEnvironment) async {
   print("git ${args.join(' ')}");