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..62e636d383c7c45fd9390c24fe19c3494a3c25ec 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -384,6 +384,18 @@ class CodeGenerator extends tree_ir.StatementVisitor
       return buildStaticHelperInvocation(
           glue.getCheckSubtype(),
           [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;
+
+      // We use the helper:

[karlklose, 2015/05/28 07:42:30]

I am not sure these comments have much value. Maybe we should add documentation
to glue, where the helpers are located?

[asgerf, 2015/05/28 08:52:55]

I added the doc comments to glue and removed this instance of the comment.

I decided to keep the one for checkSubtype since it gives meaning to some
variable names and explains why some things are nulled.

+      //
+      //     checkSubtypeOfRuntime(value, runtimeType)
+      //
+      return buildStaticHelperInvocation(
+          glue.getCheckSubtypeOfRuntime(),
+          [value, typeValue]);

[karlklose, 2015/05/28 07:42:30]

Add type arguments to list (also in line 386.)

[asgerf, 2015/05/28 08:52:55]

Done.

     }
     return giveup(node, 'type check unimplemented for $type.');
   }