Additional samples for Concurrency in Dart.

• bin/send_and_receive.dart: Demonstrates how to spawn a worker Isolate with Isolate.run. Isolate.run executes the specified function and returns the result to the main isolate efficiently using Isolate.exit which avoids copying.
• 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.