Create destination directory before git cloning into it.

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 e0737e8e02c795679683f21e30f165a3ea8e4581..ca3bb712ef1af6a566a59ff19707c29036b27040 100644
--- a/utils/pub/git_source.dart
+++ b/utils/pub/git_source.dart
@@ -153,9 +153,13 @@ class GitSource extends Source {
    * remote repository. See the manpage for `git clone` for more information.
    */
   Future _clone(String from, String to, [bool mirror=false]) {
-    var args = ["clone", from, to];
-    if (mirror) args.insertRange(1, 1, "--mirror");
-    return runGit(args).transform((result) {
+    // Git on Windows does not seem to automatically create the destination
+    // directory.
+    return ensureDir(to).chain((_) {
+      var args = ["clone", from, to];
+      if (mirror) args.insertRange(1, 1, "--mirror");
+      return runGit(args);
+    }).transform((result) {
       if (!result.success) throw 'Git failed.';
       return null;
     });