Ensure that renames fail atomically in pub.

R=rnystrom@google.com
BUG=12436

Review URL: https://codereview.chromium.org//22799008

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@26150 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/io.dart

@@ -309,7 +309,13 @@ void cleanDir(String dir) {
 /// Renames (i.e. moves) the directory [from] to [to].
 void renameDir(String from, String to) {
   log.io("Renaming directory $from to $to.");
-  new Directory(from).renameSync(to);
+  try {
+    new Directory(from).renameSync(to);
+  } on IOException catch (error) {
+    // Ensure that [to] isn't left in an inconsistent state. See issue 12436.
+    if (entryExists(to)) deleteEntry(to);
+    rethrow;
+  }
 }
 
 /// Creates a new symlink at path [symlink] that points to [target]. Returns a