Make regexp flag parsing stricter.

src/messages.js

 // Copyright 2006-2008 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 189 matching lines @@
       handler_returned_undefined:   ["Proxy handler ", "%0", " returned undefined for '", "%1", "' trap"],
       proxy_prop_not_configurable:  ["Trap ", "%1", " of proxy handler ", "%0", " returned non-configurable descriptor for property ", "%2"],
       proxy_non_object_prop_names:  ["Trap ", "%1", " returned non-object ", "%0"],
       proxy_repeated_prop_name:     ["Trap ", "%1", " returned repeated property name ", "%2"],
       invalid_weakmap_key:          ["Invalid value used as weak map key"],
       // RangeError
       invalid_array_length:         ["Invalid array length"],
       stack_overflow:               ["Maximum call stack size exceeded"],
       // SyntaxError
       unable_to_parse:              ["Parse error"],
-      duplicate_regexp_flag:        ["Duplicate RegExp flag ", "%0"],
+      invalid_regexp_flags:         ["Invalid flags supplied to RegExp constructor '", "%0", "'"],
       invalid_regexp:               ["Invalid RegExp pattern /", "%0", "/"],
       illegal_break:                ["Illegal break statement"],
       illegal_continue:             ["Illegal continue statement"],
       illegal_return:               ["Illegal return statement"],
       error_loading_debugger:       ["Error loading debugger"],
       no_input_to_regexp:           ["No input to ", "%0"],
       invalid_json:                 ["String '", "%0", "' is not valid JSON"],
       circular_structure:           ["Converting circular structure to JSON"],
       obj_ctor_property_non_object: ["Object.", "%0", " called on non-object"],
       called_on_null_or_undefined:  ["%0", " called on null or undefined"],
@@ skipping 900 matching lines @@
     else throw e;
   }
 }
 
 
 InstallFunctions($Error.prototype, DONT_ENUM, ['toString', errorToString]);
 
 // Boilerplate for exceptions for stack overflows. Used from
 // Isolate::StackOverflow().
 const kStackOverflowBoilerplate = MakeRangeError('stack_overflow', []);