Add Flutter SDK v0.3.1

[ogaclejapan]

After making all changes to the cask:

• brew cask audit --download {{cask_file}} is error-free.
• brew cask style --fix {{cask_file}} reports no offenses.
• The commit message includes the cask’s name and version.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• brew cask install {{cask_file}} worked successfully.
• brew cask uninstall {{cask_file}} worked successfully.
• Checked there are no open pull requests for the same cask.
• Checked the cask was not already refused in closed issues.
• Checked the cask is submitted to the correct repo.

Flutter SDK is only available as beta.
There is no stable version, so I sent PR to this repository.

https://flutter.io/setup-macos/

[vitorgalvao]

This seems to be a CLI-only open-source tool. Please submit it first to Homebrew/core and only if they don’t accept it, submit here (linking back to the pull request where it was refused).

This policy is documented in faq/rejected_casks:

The app is both open-source and CLI-only (i.e. it only uses the binary artifact). In that case, and in the spirit of deduplication, submit it first to Homebrew/core. If it is rejected there, you may then try again in Homebrew-Cask (link us to the issue on Homebrew so we can see their reasoning for rejection).

[ogaclejapan]

@vitorgalvao Thank you for your review.

I tried to create formula for homebrew/core.

class Flutter < Formula
  desc "Flutter is Google’s mobile app SDK for crafting high-quality native interfaces on iOS and Android in record time. "
  homepage "https://flutter.io/"
  url "https://storage.googleapis.com/flutter_infra/releases/beta/macos/flutter_macos_v0.3.1-beta.zip"
  sha256 "a88356eb37c37e89f92698ed06eaf02d6364c50f6accea6fe793b9d04c9e0c98"

  bottle :unneeded

  def install
    libexec.install Dir["*"]

    %w[flutter].each do |tool|
      (bin/tool).write <<~EOS
        #!/bin/bash
        TOOL="#{libexec}/bin/#{tool}"
        exec "$TOOL" "$@"
      EOS
    end
  end

Flutter's binary strongly depends on Git repository, so does not work.
https://github.com/flutter/flutter/blob/master/bin/flutter#L143-L150

~$ flutter doctor
Error: The Flutter directory is not a clone of the GitHub project.
       The flutter tool requires Git in order to operate properly;
       to set up Flutter, run the following command:
       git clone -b beta https://github.com/flutter/flutter.git

The '.git' directory is excluded when install formula, so execution has failed.

~$ ls -a1 /usr/local/Cellar/flutter/0.3.1-beta/libexec/
.
..
CONTRIBUTING.md
ISSUE_TEMPLATE.md
PATENTS
analysis_options.yaml
analysis_options_repo.yaml
appveyor.yml
bin
dev
examples
flutter_console.bat
packages
version

In the current Flutter's binary file, it seems that will not work on homebrew/core.