Make yield* throw an error if not yielding an Iterable

runtime/lib/core_patch.dart

 // Copyright (c) 2013, 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.
 
 import "dart:math";
 import "dart:typed_data";
 
 // We need to pass the exception and stack trace objects as second and third
 // parameter to the continuation.  See vm/ast_transformer.cc for usage.
 void  _asyncCatchHelper(catchFunction, continuation) {
@@ skipping 46 matching lines @@
           return true;
         }
         yieldEachIterator = null;
       }
       isYieldEach = false;
       if (!moveNextFn(this)) {
         moveNextFn = null;
         current = null;
         return false;
       }
-      if (isYieldEach && (current is Iterable)) {
-        yieldEachIterator = current.iterator;
+      if (isYieldEach) {
+        // Spec mandates: it is a dynamic error if the class of [the object
+        // returned by yield*] does not implement Iterable.
+        yieldEachIterator = (current as Iterable).iterator;
         continue;
       }
       return true;    
     }
   }
 }