Isolate.spawn{Uri} only reports errors asynchronously.

runtime/lib/isolate_patch.dart

Index: runtime/lib/isolate_patch.dart
diff --git a/runtime/lib/isolate_patch.dart b/runtime/lib/isolate_patch.dart
index 3792a5b4d2ddbdee1d72446d80c67cae5cd82062..7d185beb17b9c3c9ef317d235d68cde73cb59718 100644
--- a/runtime/lib/isolate_patch.dart
+++ b/runtime/lib/isolate_patch.dart
@@ -229,40 +229,39 @@ void _startIsolate(Function entryPoint, bool isSpawnUri) {
 patch class Isolate {
   /* patch */ static Future<Isolate> spawn(
       void entryPoint(message), var message) {
-    Completer completer = new Completer<Isolate>.sync();
     try {
       // The VM will invoke [_startIsolate] with entryPoint as argument.
       SendPort controlPort = _spawnFunction(entryPoint);
       RawReceivePort readyPort = new RawReceivePort();
       controlPort.send([readyPort.sendPort, message]);
+      Completer completer = new Completer<Isolate>.sync();
       readyPort.handler = (readyMessage) {
         assert(readyMessage == 'started');
         readyPort.close();
         completer.complete(new Isolate._fromControlPort(controlPort));
       };
-    } catch(e, st) {
-      // TODO(14718): we want errors to go into the returned future.
-      rethrow;
+      return completer.future;
+    } catch (e, st) {
+      return new Future<Isolate>.error(e, st);
     };
-    return completer.future;
   }
 
   /* patch */ static Future<Isolate> spawnUri(
       Uri uri, List<String> args, var message) {
-    Completer completer = new Completer<Isolate>.sync();
     try {
       // The VM will invoke [_startIsolate] and not `main`.
       SendPort controlPort = _spawnUri(uri.toString());
       RawReceivePort readyPort = new RawReceivePort();
       controlPort.send([readyPort.sendPort, args, message]);
+      Completer completer = new Completer<Isolate>.sync();
       readyPort.handler = (readyMessage) {
         assert(readyMessage == 'started');
         readyPort.close();
         completer.complete(new Isolate._fromControlPort(controlPort));
       };
-    } catch(e, st) {
-      // TODO(14718): we want errors to go into the returned future.
-      rethrow;
+      return completer.future;
+    } catch (e, st) {
+      return new Future<Isolate>.error(e, st);
     };
     return completer.future;
   }