Ensure that failure to start an isolate all end up in the future.

runtime/lib/isolate_patch.dart

Index: runtime/lib/isolate_patch.dart
diff --git a/runtime/lib/isolate_patch.dart b/runtime/lib/isolate_patch.dart
index add3c5143f117c404d8a693aec1abd308b6522f7..d501ce1b07baa9f370b5c90d68970dba3edd091a 100644
--- a/runtime/lib/isolate_patch.dart
+++ b/runtime/lib/isolate_patch.dart
@@ -200,24 +200,36 @@ void _startIsolate(Function entryPoint, bool isSpawnUri) {
     keepAlivePort.close();
 
     SendPort replyTo = message[0];
-    if (replyTo != null) {
-      // TODO(floitsch): don't send ok-message if we can't find the entry point.
-      replyTo.send("started");
-    }
     if (isSpawnUri) {
       assert(message.length == 3);
       List<String> args = message[1];
       var isolateMessage = message[2];
-      if (entryPoint is _MainFunctionArgsMessage) {
+      if (entryPoint == null) {
+        // Set to null when lookup of "main" failed in C++ code.
+        if (replyTo != null) {
+          replyTo.send(["error",
+                        "No main method in isolate library"]);
+        }
+      } else if (entryPoint is _MainFunctionArgsMessage) {
+        if (replyTo != null) replyTo.send("started");
         entryPoint(args, isolateMessage);
       } else if (entryPoint is _MainFunctionArgs) {
+        if (replyTo != null) replyTo.send("started");
         entryPoint(args);
-      } else {
+      } else if (entryPoint is _MainFunction) {
+        if (replyTo != null) replyTo.send("started");
         entryPoint();
+      } else {
+        // Report error back to spawner.
+        if (replyTo != null) {
+          replyTo.send(["error",
+                        "Incorrect parameter count on main: $entryPoint"]);
+        }
       }
     } else {
       assert(message.length == 2);
       var entryMessage = message[1];
+      if (replyTo != null) replyTo.send("started");
       entryPoint(entryMessage);
     }
   }
@@ -256,9 +268,15 @@ patch class Isolate {
       controlPort.send([readyPort.sendPort, args, message]);
       Completer completer = new Completer<Isolate>.sync();
       readyPort.handler = (readyMessage) {
-        assert(readyMessage == 'started');
         readyPort.close();
-        completer.complete(new Isolate(controlPort));
+        if ('started' == readyMessage) {
+          completer.complete(new Isolate(controlPort));
+        } else {
+          assert(readyMessage is List && readyMessage[0] == "error");
+          String error = readyMessage[1];
+          var remoteError = new IsolateSpawnException(error);
+          completer.completeError(remoteError);
+        }
       };
       return completer.future;
     } catch (e, st) {