Implement is-checks against type variables.

sdk/lib/_internal/compiler/implementation/enqueue.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 dart2js;
 
 class EnqueueTask extends CompilerTask {
   final ResolutionEnqueuer resolution;
   final CodegenEnqueuer codegen;
 
@@ skipping 334 matching lines @@
   void registerFieldGetter(Element element) {
     universe.fieldGetters.add(element);
   }
 
   void registerFieldSetter(Element element) {
     universe.fieldSetters.add(element);
   }
 
   void registerIsCheck(DartType type) {
     universe.isChecks.add(type);
+    if (type.kind == TypeKind.TYPE_VARIABLE) {
+      compiler.hasVariableTypecheck = true;
+    }
     compiler.backend.registerIsCheck(type, this);
   }
 
   void registerAsCheck(DartType type) {
     registerIsCheck(type);
     compiler.backend.registerAsCheck(type);
   }
 
   void forEach(f(WorkItem work));
 
@@ skipping 164 matching lines @@
     while(!queue.isEmpty) {
       // TODO(johnniwinther): Find an optimal process order for codegen.
       f(queue.removeLast());
     }
   }
 
   void _logSpecificSummary(log(message)) {
     log('Compiled ${generatedCode.length} methods.');
   }
 }