Dart: Begins moving bindings and tests to the embedder.

This CL also:
- Uses snapshot uri mapping to load the builtin library and
  the mojo libraries into the snapshot with "dart:" names.

- Removes file IO code from Builtin because the built-in
  libraries must always only be read from the snapshot.

- Loads the initial script using functions from builtin.dart
  so that the initial script may be a script snapshot.

- Removes the MojoType<T> interface because using a type
  parameter in a static is not allowed.

- Adds core_standalone.dart and bindings_standalone.dart for
  use by tests that I have not yet moved over to run in the
  embedder.

I can't move all of the tests over yet because they rely on
async/await, which relies on the timer factory closure being
set up, which will be complex enough for its own CL.

BUG=
R=asiva@google.com, jamesr@chromium.org

Review URL: https://codereview.chromium.org/749623004