Add Null class to dart:core.

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 565 matching lines @@
       } else {
         // We cannot just return false, because the expression may be of
         // type int or double.
       }
     // We 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 (!RuntimeTypes.hasTypeArguments(type)) {
       TypeMask expressionMask = expressionType.computeMask(compiler);
-      TypeMask typeMask = new TypeMask.nonNullSubtype(type);
+      TypeMask typeMask = (element == compiler.nullClass)
+          ? new TypeMask.subtype(type) : new TypeMask.nonNullSubtype(type);
       if (expressionMask.union(typeMask, compiler) == typeMask) {
         return graph.addConstantBool(true, compiler);
       } else if (expressionMask.intersection(typeMask, compiler).isEmpty) {
         return graph.addConstantBool(false, compiler);
       }
     }
     return node;
   }
 
   HInstruction visitTypeConversion(HTypeConversion node) {
@@ skipping 741 matching lines @@
       // that knows it is not of a specific Type.
     }
 
     for (HIf ifUser in notIfUsers) {
       changeUsesDominatedBy(ifUser.elseBlock, input, convertedType);
       // TODO(ngeoffray): Also change uses for the then block on a HType
       // that knows it is not of a specific Type.
     }
   }
 }