Add regression test for r24720.

sdk/lib/_internal/compiler/implementation/ssa/builder.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 ssa;
 
 /**
  * A special element for the extra parameter taken by intercepted
  * methods. We need to override [Element.computeType] because our
  * optimizers may look at its declared type.
@@ skipping 1329 matching lines @@
             "Parameters and arguments didn't match for super/redirect call",
             element: constructor);
       }
 
       ClassElement superclass = constructor.getEnclosingClass();
       if (backend.classNeedsRti(superclass)) {
         // If [superclass] needs RTI, we have to give a value to its
         // type parameters. Those values are in the [supertype]
         // declaration of [subclass].
         ClassElement subclass = inlinedFromElement.getEnclosingClass();
+        // If [inlinedFromElement] is a generative constructor then [superclass] 
+        // is a superclass of [subclass]. If [inlinedFromElement] is a 
+        // redirecting constructor then [superclass] is the same as [subclass]. 
+        // Using [DartType.asInstanceOf] handles both these cases.
         InterfaceType supertype = subclass.thisType.asInstanceOf(superclass);
         Link<DartType> typeVariables = superclass.typeVariables;
         supertype.typeArguments.forEach((DartType argument) {
           localsHandler.updateLocal(typeVariables.head.element,
               analyzeTypeArgument(argument, callNode));
           typeVariables = typeVariables.tail;
         });
         // If the supertype is a raw type, we need to set to null the
         // type variables.
         assert(typeVariables.isEmpty
@@ skipping 4077 matching lines @@
           new HSubGraphBlockInformation(elseBranch.graph));
 
     HBasicBlock conditionStartBlock = conditionBranch.block;
     conditionStartBlock.setBlockFlow(info, joinBlock);
     SubGraph conditionGraph = conditionBranch.graph;
     HIf branch = conditionGraph.end.last;
     assert(branch is HIf);
     branch.blockInformation = conditionStartBlock.blockFlow;
   }
 }