Fix a potential crash in const declaration.

test/mjsunit/regress/regress-88591.js

+// Copyright 2011 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
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+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
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+//       with the distribution.
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+//       contributors may be used to endorse or promote products derived
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+//
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+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Regression test for a crash.  A data property in the global object's
+// prototype shadowed by a setter in the global object's prototype's
+// prototype would crash or assert when seen by Runtime_DeclareContextSlot.
+var called = false;
+Object.prototype.__defineSetter__('x', function(x) { called = true; });
+Object.prototype.__defineGetter__('x', function () { return 0; });
+
+this.__proto__ = { x: 1 };
+
+try { fail; } catch (e) { eval('const x = 2'); }
+
+var o = Object.getOwnPropertyDescriptor(this, 'x');
+assertFalse(called);
+assertEquals(2, o.value);
+assertEquals(false, o.writable);