Add Null class to dart:core.

sdk/lib/_internal/compiler/implementation/ssa/codegen.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;
 
 class SsaCodeGeneratorTask extends CompilerTask {
 
   final JavaScriptBackend backend;
 
@@ skipping 2328 matching lines @@
     bool negative = relation == '!==';
 
     if (node.isVariableCheck || node.isCompoundCheck) {
       use(node.checkCall);
       if (negative) push(new js.Prefix('!', pop()));
     } else {
       assert(node.isRawCheck);
       LibraryElement coreLibrary = compiler.coreLibrary;
       ClassElement objectClass = compiler.objectClass;
       Element element = type.element;
-
-      if (identical(element, objectClass) || type.treatAsDynamic) {
+      if (element == compiler.nullClass) {
+        if (negative) {
+          checkNonNull(input);
+        } else {
+          checkNull(input);
+        }
+      } else if (identical(element, objectClass) || type.treatAsDynamic) {
         // The constant folder also does this optimization, but we make
         // it safe by assuming it may have not run.
         push(newLiteralBool(!negative), node);
       } else if (element == compiler.stringClass) {
         checkString(input, relation);
         attachLocationToLast(node);
       } else if (element == compiler.doubleClass) {
         checkDouble(input, relation);
         attachLocationToLast(node);
       } else if (element == compiler.numClass) {
@@ skipping 623 matching lines @@
   if (leftType.canBeNull() && rightType.canBeNull()) {
     if (left.isConstantNull() || right.isConstantNull() ||
         (leftType.isPrimitive(compiler) && leftType == rightType)) {
       return '==';
     }
     return null;
   } else {
     return '===';
   }
 }