Do not generate a duplicate ExternalName class

tools/patch_sdk.dart

 #!/usr/bin/env dart
 // Copyright (c) 2015, 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.
 
 /// Command line tool to merge the SDK libraries and our patch files.
 /// This is currently designed as an offline tool, but we could automate it.
 
 import 'dart:io';
 import 'dart:math' as math;
@@ skipping 164 matching lines @@
       if (needsUpdate) {
         var contents = <String>[libraryContents];
         contents.addAll(partFiles.map((f) => f.readAsStringSync()));
         if (patchExists) {
           var patchContents = patchFile.readAsStringSync();
           contents = _patchLibrary(
               patchFile.path, contents, patchContents);
         }
 
         for (var i = 0; i < outPaths.length; i++) {
-          if (path.basename(outPaths[i]) == 'internal.dart') {
-            contents[i] += '''
-
-/// Marks a function as an external implementation ("native" in the Dart VM).
-///
-/// Provides a backend-specific String that can be used to identify the
-/// function's implementation
-class ExternalName {
-  final String name;
-  const ExternalName(this.name);
-}
-''';
-          }
-
           _writeSync(outPaths[i], contents[i]);
         }
       }
     }
   }
   if (mode == 'vm') {
 
     for (var tuple in [['_builtin', 'builtin.dart']]) {
       var vmLibrary = tuple[0];
       var dartFile = tuple[1];
@@ skipping 332 matching lines @@
     if (diff != 0) return diff;
     return end - other.end;
   }
 }
 
 List<SdkLibrary> _getSdkLibraries(String contents) {
   var libraryBuilder = new SdkLibrariesReader_LibraryBuilder(true);
   parseCompilationUnit(contents).accept(libraryBuilder);
   return libraryBuilder.librariesMap.sdkLibraries;
 }