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..da660ba917a166df0ec140b5b26dd5ac9b854dbe 100644
--- a/runtime/lib/print_patch.dart
+++ b/runtime/lib/print_patch.dart
@@ -8,10 +8,15 @@ patch void printToConsole(String line) {
   _printClosure(line);
 }
 
-void _unsupportedPrint(Object obj) {
+void _unsupportedPrint(String line) {
   throw new UnsupportedError("'print' is not supported");
 }
 
+// Ignores printed lines.
+_PrintClosure _nullPrintClosure = (String line) {
+  /* do nothing. */
+};
+
 // _printClosure can be overwritten by the embedder to supply a different
 // print implementation.
 _PrintClosure _printClosure = _unsupportedPrint;