Isolate.spawn{Uri} only reports errors asynchronously.

runtime/lib/print_patch.dart

Index: runtime/lib/print_patch.dart
diff --git a/runtime/lib/print_patch.dart b/runtime/lib/print_patch.dart
index 186b4fb8ecee7eb7ce6f7752deb1105df4fa55be..539294aff47510f2e73df6e5fdf2468990344db9 100644
--- a/runtime/lib/print_patch.dart
+++ b/runtime/lib/print_patch.dart
@@ -2,13 +2,15 @@
 // 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");
 }