Implement createDirectory in file_system_vm.dart

[munificent]

Frog's file system wrapper only supports createDirectory on node, which is what dartdoc uses. A friendly Googler sent this to support it on the VM:

void createDirectory(String path, [bool recursive = false]) {
   if (path.length == 0) {
     return;
   }
   if (new Directory(path).exists()) {
     return;
   }
   if (recursive) {
     // TODO(): replace with path separator reported from vm/hostOS.
     // Needs to be added to vm/runtime/directory.cc & .dart.
     final String pathSeparatorPattern = "/";

     List<String> parts = path.split(pathSeparatorPattern);
     String partialPath = parts[0];
     for (int i = 1; i < parts.length; i++) {
       partialPath += partialPath + pathSeparatorPattern + parts[i];
       new Directory(partialPath).create();
     }
   } else {
     new Directory(path).create();
   }
}

Should be easy to drop in, but we really need some tests for this first.

[whesse]

There are a few problems here:
Directory.createSyuc() throws an exception if the directory exists already, so we need to check for that.

The path would need to be normalized ( .. and . resolved ) before running this algorithm. Especially consider "../make_a_sibling/../another_sibling/in_here"

But yes, we do need this function (recursive create directory), and will probably implement it.

[anders-sandholm]

Removed Area-Frog label.
Added Area-Dart2JS, FromAreaFrog labels.

[kasperl]

Added WontFix label.