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);
+// A print-closure gets a String that should be printed. In general the
+// string is a line, but it may contain "\n" characters.
+typedef void _PrintClosure(String line);
 
 patch void printToConsole(String line) {
   _printClosure(line);
 }
 
-void _unsupportedPrint(Object obj) {
+void _unsupportedPrint(String line) {
   throw new UnsupportedError("'print' is not supported");
 }
 
 // _printClosure can be overwritten by the embedder to supply a different
 // print implementation.
 _PrintClosure _printClosure = _unsupportedPrint;