Isolate.spawn{Uri} only reports errors asynchronously.

sdk/lib/_internal/lib/isolate_patch.dart

Index: sdk/lib/_internal/lib/isolate_patch.dart
diff --git a/sdk/lib/_internal/lib/isolate_patch.dart b/sdk/lib/_internal/lib/isolate_patch.dart
index 2fa1b2b5905f38431687d83d41efbd3198949fbf..8a43785ab537a1cc8f4e583e01ebea4f29f935d4 100644
--- a/sdk/lib/_internal/lib/isolate_patch.dart
+++ b/sdk/lib/_internal/lib/isolate_patch.dart
@@ -12,23 +12,33 @@ import 'dart:_isolate_helper' show IsolateNatives,
 
 patch class Isolate {
   patch static Future<Isolate> spawn(void entryPoint(message), var message) {
-    SendPort controlPort = IsolateNatives.spawnFunction(entryPoint, message);
-    return new Future<Isolate>.value(new Isolate._fromControlPort(controlPort));
+    try {
+      SendPort controlPort = IsolateNatives.spawnFunction(entryPoint, message);
+      return new Future<Isolate>.value(
+          new Isolate._fromControlPort(controlPort));
+    } catch (e, st) {
+      return new Future<Isolate>.error(e, st);
+    }
   }
 
   patch static Future<Isolate> spawnUri(
       Uri uri, List<String> args, var message) {
-    if (args is List<String>) {
-      for (int i = 0; i < args.length; i++) {
-        if (args[i] is! String) {
-          throw new ArgumentError("Args must be a list of Strings $args");
+    try {
+      if (args is List<String>) {
+        for (int i = 0; i < args.length; i++) {
+          if (args[i] is! String) {
+            throw new ArgumentError("Args must be a list of Strings $args");
+          }
         }
+      } else if (args != null) {
+        throw new ArgumentError("Args must be a list of Strings $args");
       }
-    } else if (args != null) {
-      throw new ArgumentError("Args must be a list of Strings $args");
+      SendPort controlPort = IsolateNatives.spawnUri(uri, args, message);
+      return new Future<Isolate>.value(
+          new Isolate._fromControlPort(controlPort));
+    } catch (e, st) {
+      return new Future<Isolate>.error(e, st);
     }
-    SendPort controlPort = IsolateNatives.spawnUri(uri, args, message);
-    return new Future<Isolate>.value(new Isolate._fromControlPort(controlPort));
   }
 }