[generators] Make runtime functions more robust.

test/mjsunit/regress/regress-618657.js

Index: test/mjsunit/regress/regress-618657.js
diff --git a/test/mjsunit/regress/regress-618657.js b/test/mjsunit/regress/regress-618657.js
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index 0000000000000000000000000000000000000000..7bf84694301648bb492a499a3a11fa854b9c22b5
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+++ b/test/mjsunit/regress/regress-618657.js
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+// 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: --ignition --ignition-generators --ignition-filter=-foo
+
+function* foo() { yield 42 }
+var g = foo();
+assertEquals(42, g.next().value);
+assertEquals(true, g.next().done);