Make pub handle missing git more gracefully.

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
index cc08e39113482b6a606d53ac5747e51fa1df1762..f2cfa025ed3022f5df89b9a3a84bb335a344baf5 100644
--- a/utils/pub/git_source.dart
+++ b/utils/pub/git_source.dart
@@ -22,9 +22,18 @@ class GitSource extends Source {
    * 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.");
+      }
+    });
   }
 
   /**