Add a test and improve comment on generic checks.

sdk/lib/_internal/compiler/implementation/ssa/optimize.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/optimize.dart	(revision 19536)
+++ sdk/lib/_internal/compiler/implementation/ssa/optimize.dart	(working copy)
@@ -541,7 +541,10 @@
       } else {
         return graph.addConstantBool(false, constantSystem);
       }
-    // TODO(karlklose): remove the hasTypeArguments check.
+    // Wee need the [:hasTypeArguments:] check because we don't have
+    // the notion of generics in the backend. For example, [:this:] in
+    // a class [:A<T>:], is currently always considered to have the
+    // raw type.
     } else if (expressionType.isUseful()
                && !expressionType.canBeNull()
                && !RuntimeTypeInformation.hasTypeArguments(type)) {