Update isolate documentation.

sdk/lib/_internal/lib/isolate_helper.dart

Index: sdk/lib/_internal/lib/isolate_helper.dart
diff --git a/sdk/lib/_internal/lib/isolate_helper.dart b/sdk/lib/_internal/lib/isolate_helper.dart
index af5c9d55185f39e4c96c4ea28a46acce19f6f987..39ba791b17fcca66e9ceb8fa8876b9a9d23075f3 100644
--- a/sdk/lib/_internal/lib/isolate_helper.dart
+++ b/sdk/lib/_internal/lib/isolate_helper.dart
@@ -23,6 +23,13 @@ import 'dart:_interceptors' show JSExtendableArray;
 
 ReceivePort controlPort;
 
+class IsolateDeserializationError extends Error {
+  final String _message;
+  IsolateDeserializationError(this._message);
+
+  toString() => "IsolateDeserializationError: $_message";
+}
+
 /**
  * Called by the compiler to support switching
  * between isolates when we get a callback from the DOM.
@@ -1077,8 +1084,7 @@ class _MessageTraverser {
   visitSendPort(SendPort x);
 
   visitObject(Object x) {
-    // TODO(floitsch): make this a real exception. (which one)?
-    throw "Message serialization: Illegal value $x passed";
+    throw new IsolateMessageException("Illegal value $x passed");

[Lasse Reichstein Nielsen, 2013/12/11 10:38:28]

Could we put the value of x into the object?
If the receiver of the exception is also the one passing the value, it won't
leak anything, and it can be useful to inspect the object that causes the
problem.
This is an Exception, so you need data that can allow you to act on the
exception. The value of x, and perhaps the entire message as well, seems
reasonable.

[floitsch, 2013/12/11 12:14:16]

Yes. I would like that.
The problem is that it would require more changes to the VM, so I only marked it
as TODO.

   }
 
   static bool isPrimitive(x) {
@@ -1233,8 +1239,7 @@ class _Deserializer {
   deserializeSendPort(List x);
 
   deserializeObject(List x) {
-    // TODO(floitsch): Use real exception (which one?).
-    throw "Unexpected serialized object";
+    throw new IsolateDeserializationError("Unexpected serialized object");

[Lasse Reichstein Nielsen, 2013/12/11 10:38:28]

How does this error occur?

It happens during deserialization, but anything passed through serialization
should be deserializable as well, so is this really a sanity check?

[floitsch, 2013/12/11 12:14:16]

It's a sanity check, and could happen once we have cross-computer isolates. In
that case other users could fake isolate-messages.

   }
 }