Implement is-checks against type variables.

sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart

 // Copyright (c) 2012, 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 dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 199 matching lines @@
     }
     return true;
   }
 
   DartBackend(Compiler compiler, List<String> strips)
       : tasks = <CompilerTask>[],
         forceStripTypes = strips.indexOf('types') != -1,
         stripAsserts = strips.indexOf('asserts') != -1,
         super(compiler);
 
+  void addBackendRtiDependencies(World world) {}
+
   void enqueueHelpers(ResolutionEnqueuer world) {
     // Right now resolver doesn't always resolve interfaces needed
     // for literals, so force them. TODO(antonm): fix in the resolver.
     final LITERAL_TYPE_NAMES = const [
       'Map', 'List', 'num', 'int', 'double', 'bool'
     ];
     final coreLibrary = compiler.coreLibrary;
     for (final name in LITERAL_TYPE_NAMES) {
       ClassElement classElement = coreLibrary.findLocal(new SourceString(name));
       classElement.ensureResolved(compiler);
@@ skipping 353 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().canonicalUri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Iterable<Element> elements) =>
     sorted(elements, compareElements);