ParserBase should accept ESCAPED_STRICT_RESERVED_WORD as an identifier

test/mjsunit/regress/regress-5692.js

Index: test/mjsunit/regress/regress-5692.js
diff --git a/test/mjsunit/regress/regress-5692.js b/test/mjsunit/regress/regress-5692.js
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+// 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.
+
+// "let" in non-strict mode can be a label, even if composed of unicode escape
+// sequences.
+
+var wasTouched = false;
+l\u0065t:
+do {
+  break l\u0065t;
+  wasTouched = true;
+} while (false);
+// Verify that in addition to no exception thrown, breaking to the label also
+// works.
+assertFalse(wasTouched);