Use an explicit 'this' parameter instead of 'This' nodes.

pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
index ef33a135f4deaf41d3d735f9f4bd12fb948c0ff8..3398009853ad21c179833ff33483ca675b0dfe59 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
@@ -1276,7 +1276,7 @@ class DartIrBuilderVisitor extends IrBuilderVisitor {
   @override
   ir.Primitive buildReifyTypeVariable(ir.Primitive target,
                                       TypeVariableType variable) {
-    assert(target is ir.This);
+    assert(target == irBuilder.state.thisParameter);
     ir.Primitive prim = new ir.ReifyTypeVar(variable.element);
     irBuilder.add(new ir.LetPrim(prim));
     return prim;