Additional samples for Concurrency in Dart.

• bin/send_and_receive.dart: Demonstrates how to spawn an Isolate and send it initial input using Isolate.spawn. The spawned isolate is shortlived and returns its result to the main isolate using Isolate.exit, which is fast.
• bin/long_running_isolate.dart: Similar to send_and_receive.dart, except that the spawned isolate is long running, responds to incoming messages from the main isolate, and sends responses until it receives a null over its SendPort.