Add 2.15 Formula (#105)

Co-authored-by: Alexander Thomas <athom@google.com>
diff --git a/Formula/dart@2.15.rb b/Formula/dart@2.15.rb
new file mode 100644
index 0000000..b63bbc9
--- /dev/null
+++ b/Formula/dart@2.15.rb
@@ -0,0 +1,52 @@
+# typed: false
+# frozen_string_literal: true
+
+class DartAT215 < Formula
+  desc "Dart SDK"
+  homepage "https://dart.dev"
+
+  keg_only :versioned_formula
+  if OS.mac?
+    url "https://storage.googleapis.com/dart-archive/channels/stable/release/2.15.1/sdk/dartsdk-macos-x64-release.zip"
+    sha256 "b3a6e4c21da6e4a751aff5b1597c08b4ef58683a69fc7237c639d2d941e637e2"
+  elsif OS.linux? && Hardware::CPU.intel?
+    if Hardware::CPU.is_64_bit?
+      url "https://storage.googleapis.com/dart-archive/channels/stable/release/2.15.1/sdk/dartsdk-linux-x64-release.zip"
+      sha256 "0f45dcaa53b4090b69b277b885ea9a4cb3a41589c9119113e1b978ad55ce335f"
+    else
+      url "https://storage.googleapis.com/dart-archive/channels/stable/release/2.15.1/sdk/dartsdk-linux-ia32-release.zip"
+      sha256 "491ab94edc52f9bc02a95c5ad14d99867f09d56b66e0e9eafb7b904bef79d6cc"
+    end
+  elsif OS.linux? && Hardware::CPU.arm?
+    if Hardware::CPU.is_64_bit?
+      url "https://storage.googleapis.com/dart-archive/channels/stable/release/2.15.1/sdk/dartsdk-linux-arm64-release.zip"
+      sha256 "8836c294234352cc53e8aea4a1ce0442ebbb769a536ce7f309579da5020a2395"
+    else
+      url "https://storage.googleapis.com/dart-archive/channels/stable/release/2.15.1/sdk/dartsdk-linux-arm-release.zip"
+      sha256 "751935fc08dec2121410c3f2f33de8215d8a4e5f21192a4c42c4b81dd00f8659"
+    end
+  end
+
+  def install
+    libexec.install Dir["*"]
+    bin.install_symlink "#{libexec}/bin/dart"
+    bin.write_exec_script Dir["#{libexec}/bin/{pub,dart?*}"]
+  end
+
+  def caveats
+    <<~EOS
+      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