Add a Pub source that checks packages out from Git.

utils/pub/git_source.dart

Index: utils/pub/git_source.dart
diff --git a/utils/pub/git_source.dart b/utils/pub/git_source.dart
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+++ b/utils/pub/git_source.dart
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+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/**
+ * A package source that installs packages from Git repos.
+ */
+class GitSource extends Source {
+  final String name = "git";
+
+  // TODO(nweiz): this should be cached since it uses the network, but until we
+  // support versions there's no good way to distinguish between different
+  // checkouts of the same repository.
+  final bool shouldCache = false;
+
+  GitSource();
+
+  /**
+   * Clones a Git repo to the local filesystem.
+   */
+  Future<bool> install(PackageId id, String destPath) {
+    return runProcess("git", ["clone", "--progress", id.fullName, destPath],
+        pipeStdout: true, pipeStderr: true).
+      transform((result) => result.success);
+  }
+
+  /**
+   * The package name of a Git repo is the name of the directory into which
+   * it'll be cloned.
+   */
+  String packageName(PackageId id) =>
+      basename(id.fullName).replaceFirst(const RegExp("\.git\$"), "");
+}