Adds an ApplicationConnection class to Dart
Adds an ApplicationConnection class to Dart

At this time ApplicationConnection only contains the
ServiceProviderProxy for the remote side. It needs to contain a
ServiceProvider for exposing services to the remote side as well.

This mirrors how the C++ side represents connections to remote
applications. Although the C++ side caches ApplicationConnections to
remote urls and this code doesn't.

R=zra@google.com, zra@chromium.org

Review URL: https://codereview.chromium.org/907953004

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