Make yield* throw an error if not yielding an Iterable

tests/language/sync_generator1_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Simple test program for sync* generator functions.
 
 // VMOptions=--optimization_counter_threshold=10
 
 import "package:expect/expect.dart";
 
@@ skipping 17 matching lines @@
     var e = start;
     while (e <= end) yield e++;
   }
 }
 
 get sync sync* {  // sync is a legal identifier.
   yield "sync";
 }
 
 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();
     print(sums);
     Expect.isTrue(sums is Iterable);
     Expect.equals(10, sums.length);
     Expect.equals(1, sums.first);
     Expect.equals(55, sums.last);
     var q = "";
     for (var n in sums.take(3)) {
@@ skipping 12 matching lines @@
     elems2.forEach((e) {
       Expect.isTrue(i.moveNext());
       Expect.equals(e, i.current);
     });
 
     print(sync);
     Expect.equals("sync", sync.single);
 
     print(einsZwei());
     Expect.equals("(1, 2, 3, 5, [6])", einsZwei().toString());
+
+    Expect.throws(() => dreiVier().toString());
   }
 }