dart2js cps: 'is'-check directly against type variables.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index c133788a1f9a2fa81d5a5ebb791b7f7702173bc7..6379605fcfafc158db3b7d58e526114ac8585bb8 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -383,7 +383,14 @@ class CodeGenerator extends tree_ir.StatementVisitor
 
       return buildStaticHelperInvocation(
           glue.getCheckSubtype(),
-          [value, isT, typeArgumentArray, asT]);
+          <js.Expression>[value, isT, typeArgumentArray, asT]);
+    } else if (type is TypeVariableType) {
+      glue.registerIsCheck(type, registry);
+      // The only type argument is the type held in the type variable.
+      js.Expression typeValue = typeArguments.single;
+      return buildStaticHelperInvocation(
+          glue.getCheckSubtypeOfRuntime(),
+          <js.Expression>[value, typeValue]);
     }
     return giveup(node, 'type check unimplemented for $type.');
   }