Fix braino in SSA optimizer and add a test for it.

sdk/lib/_internal/compiler/implementation/ssa/optimize.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;
 
 abstract class OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
@@ skipping 668 matching lines @@
       constantInterceptor = backend.jsIntClass;
     } else if (type.isDouble()) {
       constantInterceptor = backend.jsDoubleClass;
     } else if (type.isBoolean()) {
       constantInterceptor = backend.jsBoolClass;
     } else if (type.isString()) {
       constantInterceptor = backend.jsStringClass;
     } else if (type.isArray()) {
       constantInterceptor = backend.jsArrayClass;
     } else if (type.isNull()) {
-      constantInterceptor = backend.jsIntClass;
+      constantInterceptor = backend.jsNullClass;
     } else if (type.isNumber()) {
       // If the method being intercepted is not defined in [int] or
       // [double] we can safely use the number interceptor.
       if (!intercepted.contains(compiler.intClass)
           && !intercepted.contains(compiler.doubleClass)) {
         constantInterceptor = backend.jsNumberClass;
       }
     }
 
     if (constantInterceptor == null) return null;
@@ skipping 820 matching lines @@
     HBasicBlock block = user.block;
     block.addAfter(user, interceptor);
     block.rewrite(user, interceptor);
     block.remove(user);
 
     // The interceptor will be removed in the dead code elimination
     // phase. Note that removing it here would not work because of how
     // the [visitBasicBlock] is implemented.
   }
 }