Make stray 'return' an early error

src/messages.js

 // Copyright 2012 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:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 29 matching lines @@
   unexpected_token_identifier:   ["Unexpected identifier"],
   unexpected_reserved:           ["Unexpected reserved word"],
   unexpected_strict_reserved:    ["Unexpected strict mode reserved word"],
   unexpected_eos:                ["Unexpected end of input"],
   malformed_regexp:              ["Invalid regular expression: /", "%0", "/: ", "%1"],
   unterminated_regexp:           ["Invalid regular expression: missing /"],
   regexp_flags:                  ["Cannot supply flags when constructing one RegExp from another"],
   incompatible_method_receiver:  ["Method ", "%0", " called on incompatible receiver ", "%1"],
   multiple_defaults_in_switch:   ["More than one default clause in switch statement"],
   newline_after_throw:           ["Illegal newline after throw"],
-  redeclaration:                 ["%0", " '", "%1", "' has already been declared"],
+  label_redeclaration:           ["Label '", "%0", "' has already been declared"],
+  var_redeclaration:             ["Identifier '", "%0", "' has already been declared"],
   no_catch_or_finally:           ["Missing catch or finally after try"],
   unknown_label:                 ["Undefined label '", "%0", "'"],
   uncaught_exception:            ["Uncaught ", "%0"],
   stack_trace:                   ["Stack Trace:\n", "%0"],
   called_non_callable:           ["%0", " is not a function"],
   undefined_method:              ["Object ", "%1", " has no method '", "%0", "'"],
   property_not_function:         ["Property '", "%0", "' of object ", "%1", " is not a function"],
   cannot_convert_to_primitive:   ["Cannot convert object to primitive value"],
   not_constructor:               ["%0", " is not a constructor"],
   not_defined:                   ["%0", " is not defined"],
@@ skipping 1286 matching lines @@
     %GetAndClearOverflowedStackTrace(this);
   };
 
   %DefineOrRedefineAccessorProperty(
       boilerplate, 'stack', getter, setter, DONT_ENUM);
 
   return boilerplate;
 }
 
 var kStackOverflowBoilerplate = SetUpStackOverflowBoilerplate();