Implement is-checks against type variables.

sdk/lib/_internal/compiler/implementation/js_backend/namer.dart

 // Copyright (c) 2011, 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.
 
 part of js_backend;
 
 /**
  * Assigns JavaScript identifiers to Dart variables, class-names and members.
  */
 class Namer implements ClosureNamer {
@@ skipping 663 matching lines @@
 
   String operatorIsPrefix() => r'$is';
 
   String operatorAsPrefix() => r'$as';
 
   String operatorIs(Element element) {
     // TODO(erikcorry): Reduce from $isx to ix when we are minifying.
     return '${operatorIsPrefix()}${getName(element)}';
   }
 
+  String getNativeCheckName() => r'$nativeCheck';
+
   /*
    * Returns a name that does not clash with reserved JS keywords,
    * and also ensures it won't clash with other identifiers.
    */
   String _safeName(String name, Set<String> reserved) {
     if (reserved.contains(name) || name.startsWith(r'$')) {
       name = '\$$name';
     }
     assert(!reserved.contains(name));
     return name;
@@ skipping 62 matching lines @@
       return const SourceString(r'$or');
     } else if (value == '-') {
       return const SourceString(r'$sub');
     } else if (value == 'unary-') {
       return const SourceString(r'$negate');
     } else {
       return name;
     }
   }
 }