Add IsolateRunner as a helper around Isolate.

README.md

+# isolate
+
+The `isolate` package helps with isolates and isolate communication.
+
+The package contains individual libraries with different purposes.
+
+### Creating send ports and responding to messages.
+
+The "ports.dart" sub-library contains functionality
+for creating `SendPort`s and reacting to values sent to those ports.
+
+### Working with isolates and running functions in other isolates.
+
+The "isolaterunner.dart" sub-library introduces an `IsolateRunner` class
+that gives easy access to the `Isolate` functionality, and also
+gives a way to run new functions in the isolate repeatedly, instead of
+just on the initial `spawn` call.
+
+### A central registry for values that can be used accross isolates.
+
+The "registry.dart" sub-library provides a way to create an
+object registry, and give access to it accross different isolates.
+
+### Balancing load accross several isolates.
+
+The "loadbalancer.dart" sub-library can manage multiple `Runner` objects,
+including `IsolateRunner`, and run functions on the currently least loaded
+runner.
+
+## Features and bugs
+
+Please file feature requests and bugs at the [issue tracker][tracker].
+
+[tracker]: https://github.com/dart-lang/isolate/issues