Isolate.spawn{Uri} only reports errors asynchronously.

sdk/lib/_internal/lib/collection_dev_patch.dart

Index: sdk/lib/_internal/lib/collection_dev_patch.dart
diff --git a/sdk/lib/_internal/lib/collection_dev_patch.dart b/sdk/lib/_internal/lib/collection_dev_patch.dart
index cd766c48d6fad220319415c7617c135bdbe62373..3ebb7a3967c2d19f62e728752078553eaa9d0e1b 100644
--- a/sdk/lib/_internal/lib/collection_dev_patch.dart
+++ b/sdk/lib/_internal/lib/collection_dev_patch.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:_js_primitives' show printString;
+import 'dart:_isolate_helper' show TimerImpl;
 
 patch class Symbol implements core.Symbol {
   patch const Symbol(String name)
@@ -12,3 +13,8 @@ patch class Symbol implements core.Symbol {
 patch void printToConsole(String line) {
   printString('$line');
 }
+
+patch void scheduleImmediate(void callback()) {
+  // TODO(9002): don't use the Timer to enqueue the immediate callback.
+  new TimerImpl(0, callback);
+}