[debug] Don't notify listener of exceptions internal to a desugaring.

test/mjsunit/debug-exceptions.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --expose-debug-as debug
 
 
 Debug = debug.Debug
 
 let error = false;
@@ skipping 49 matching lines @@
 
 assertCaught(() => {
   try {
     for (var a of [1, 2, 3]) {
       try {throw a} finally {}
     }
   } catch(_) {}
 });
 
 
+// Check that an internal exception in our yield* desugaring is not observable.
+{
+  uncaught = null;
+
+  let iter = {
+    next() {return {value:42, done:false}},
+    throw() {return {done:true}}
+  };
+  let iterable = {[Symbol.iterator]() {return iter}};
+  function* f() { yield* iterable }
+
+  let g = f();
+  g.next();
+  assertEquals({value: undefined, done: true}, g.throw());
+  assertNull(uncaught);  // No exception event was generated.
+}
+
+
 assertFalse(error);