Make yield* throw an error if not yielding an Iterable

tests/language/sync_generator1_test.dart

Index: tests/language/sync_generator1_test.dart
===================================================================
--- tests/language/sync_generator1_test.dart	(revision 43603)
+++ tests/language/sync_generator1_test.dart	(working copy)
@@ -35,13 +35,17 @@
 }
 
 einsZwei() sync* {
-  yield* 1;
+  yield 1;
   yield* [2, 3];
   yield* [];
   yield 5;
   yield [6];
 }
 
+dreiVier() sync* {
+  yield* 3;  // Throws type error: yielded object is not an iterable.
+}
+
 main() {
   for (int i = 0; i < 10; i++) {
     var sums = sum10();
@@ -74,5 +78,7 @@
 
     print(einsZwei());
     Expect.equals("(1, 2, 3, 5, [6])", einsZwei().toString());
+
+    Expect.throws(() => dreiVier().toString());
   }
 }