[deoptimizer] Materialize string iterators.

test/mjsunit/regress/regress-683617.js

Index: test/mjsunit/regress/regress-683617.js
diff --git a/test/mjsunit/regress/regress-683617.js b/test/mjsunit/regress/regress-683617.js
new file mode 100644
index 0000000000000000000000000000000000000000..18da682f2ee3c0e641d8db9e5bcbfd0afee43fdd
--- /dev/null
+++ b/test/mjsunit/regress/regress-683617.js
@@ -0,0 +1,17 @@
+// Copyright 2017 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: --allow-natives-syntax
+
+global = 'n';
+function f(deopt) {
+  let it = global[Symbol.iterator]();
+  if (deopt) {
+    return it.next().value;
+  }
+}
+f();
+f();
+%OptimizeFunctionOnNextCall(f);
+assertEquals('n', f(true));