Isolate.spawn{Uri} only reports errors asynchronously.

runtime/lib/print_patch.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 typedef void _PrintClosure(Object obj);
 
 patch void printToConsole(String line) {
   _printClosure(line);

[Ivan Posva, 2013/12/12 12:11:49]

Types mismatch between parameter and print closure.

[floitsch, 2013/12/12 21:08:37]

Updated the typedef.

 }
 
-void _unsupportedPrint(Object obj) {
+void _unsupportedPrint(String line) {

[Ivan Posva, 2013/12/12 12:11:49]

This does not match the _PrintClosure type.

This whole file could use a comment explaining exactly what the print closure is
supposed to do. Does it get a String passed to it and the toString conversion is
done in the caller or is the closure responsible for that part. Asked
differently is the closure only supposed to get the characters to the user or is
it implementing the functionality of print.

[floitsch, 2013/12/12 21:08:37]

Added comment.
It is supposed to get a string.

   throw new UnsupportedError("'print' is not supported");
 }
 
+// Ignores printed lines.
+_PrintClosure _nullPrintClosure = (String line) {

[Ivan Posva, 2013/12/12 12:11:49]

ditto (types don't match)

[floitsch, 2013/12/12 21:08:37]

moved to test file.

+  /* do nothing. */
+};
+
 // _printClosure can be overwritten by the embedder to supply a different
 // print implementation.
 _PrintClosure _printClosure = _unsupportedPrint;