Make pub handle missing git more gracefully.

utils/pub/git_source.dart

 // 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(rnystrom): Git packages could in theory be cached, but that adds a
   // lot of complexity. When you install a git package, you are installing
   // and pinning to a specific commit. That means different installs of the
   // same git path but at different commits need to be disambiguated in the
   // system cache. It may also lead to a lot of garbage in the system cache.
   // For now, we are punting and simply not caching them.
   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.description, destPath],
-        pipeStdout: true, pipeStderr: true).
-      transform((result) => result.success);
+    return isGitInstalled.chain((installed) {
+      if (installed) {
+        return runProcess("git",
+            ["clone", "--progress", id.description, destPath],
+            pipeStdout: true, pipeStderr: true).
+          transform((result) => result.success);
+      } else {
+        throw new Exception(
+            "Cannot install '${id.name}' from Git (${id.description}).\n"
+            "Please ensure Git is correctly installed.");
+      }
+    });
   }
 
   /**
    * 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.description).replaceFirst(const RegExp("\.git\$"), "");
 
   /**
    * Ensures [description] is a Git URL.
    */
   void validateDescription(description) {
     if (description is! String) {
       throw new FormatException("The description must be a git URL.");
     }
   }
 }