Implement is-checks against type variables.

sdk/lib/_internal/compiler/implementation/compiler.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;
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
@@ skipping 305 matching lines @@
   static const SourceString CREATE_INVOCATION_MIRROR =
       const SourceString('createInvocationMirror');
   static const SourceString INVOKE_ON = const SourceString('invokeOn');
   static const SourceString RUNTIME_TYPE = const SourceString('runtimeType');
   static const SourceString START_ROOT_ISOLATE =
       const SourceString('startRootIsolate');
   bool enabledNoSuchMethod = false;
   bool enabledRuntimeType = false;
   bool enabledFunctionApply = false;
   bool enabledInvokeOn = false;
+  bool hasVariableTypecheck = false;
 
   Stopwatch progress;
 
   static const int PHASE_SCANNING = 0;
   static const int PHASE_RESOLVING = 1;
   static const int PHASE_COMPILING = 2;
   int phase;
 
   bool compilationFailed = false;
 
@@ skipping 812 matching lines @@
 
   void close() {}
 
   toString() => name;
 
   /// Convenience method for getting an [api.CompilerOutputProvider].
   static NullSink outputProvider(String name, String extension) {
     return new NullSink('$name.$extension');
   }
 }