Remove redundant type check.

dart/runtime/lib/symbol_patch.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 patch class Symbol {
   /* patch */ const Symbol(String name)
       : this._name = _validate(name);
 
   static final RegExp _validationPattern =
       new RegExp(r'^(?:[a-zA-Z$][a-zA-Z$0-9_]*\.)*(?:[a-zA-Z$][a-zA-Z$0-9_]*=?|'
@@ skipping 13 matching lines @@
                  r'>=|'
                  r'>|'
                  r'<=|'
                  r'<|'
                  r'&|'
                  r'\^|'
                  r'\|'
                  r')$');
 
   static _validate(String name) {
-    if (name is! String) throw new ArgumentError('name must be a String');
     if (name.isEmpty) return name;
     if (name.startsWith('_')) {
       throw new ArgumentError('"$name" is a private identifier');
     }
     if (!_validationPattern.hasMatch(name)) {
       throw new ArgumentError(
           '"$name" is not an identifier or an empty String');
     }
     return name;
   }
 }