Rect is now a dart-only type, defined in its own .dart file. I use
DartConverter when passing a Rect into C++. I also special-cased Rect in the
IDL compiler so that it's passed by value, instead of allocating a new
Rect object on the heap.
This also adds a mechanism to add custom .dart files to dart_sky.dart - used by
Rect.dart.
R=eseidel@chromium.org
Review URL: https://codereview.chromium.org/1151673002
