Add Null class to dart:core.

sdk/lib/_internal/compiler/implementation/compiler.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 dart2js;
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
@@ skipping 762 matching lines @@
     objectClass = lookupCoreClass('Object');
     boolClass = lookupCoreClass('bool');
     numClass = lookupCoreClass('num');
     intClass = lookupCoreClass('int');
     doubleClass = lookupCoreClass('double');
     stringClass = lookupCoreClass('String');
     functionClass = lookupCoreClass('Function');
     listClass = lookupCoreClass('List');
     typeClass = lookupCoreClass('Type');
     mapClass = lookupCoreClass('Map');
+    nullClass = lookupCoreClass('Null');
     stackTraceClass = lookupCoreClass('StackTrace');
     if (!missingCoreClasses.isEmpty) {
       internalErrorOnElement(coreLibrary,
           'dart:core library does not contain required classes: '
           '$missingCoreClasses');
     }
 
     // The Symbol class may not exist during unit testing.
     // TODO(ahe): It is possible that we have to require the presence
     // of Symbol as we change how we implement noSuchMethod.
     symbolClass = lookupCoreClass('Symbol');
 
     final List missingHelperClasses = [];
     ClassElement lookupHelperClass(String name) {
       ClassElement result = jsHelperLibrary.find(new SourceString(name));
       if (result == null) {
         missingHelperClasses.add(name);
       }
       return result;
     }
     jsInvocationMirrorClass = lookupHelperClass('JSInvocationMirror');
     boundClosureClass = lookupHelperClass('BoundClosure');
     closureClass = lookupHelperClass('Closure');
     dynamicClass = lookupHelperClass('Dynamic_');
-    nullClass = lookupHelperClass('Null');
     if (!missingHelperClasses.isEmpty) {
       internalErrorOnElement(jsHelperLibrary,
           'dart:_js_helper library does not contain required classes: '
           '$missingHelperClasses');
     }
 
     if (types == null) {
       types = new Types(this, dynamicClass);
     }
     backend.initializeHelperClasses();
@@ skipping 662 matching lines @@
 
   void close() {}
 
   toString() => name;
 
   /// Convenience method for getting an [api.CompilerOutputProvider].
   static NullSink outputProvider(String name, String extension) {
     return new NullSink('$name.$extension');
   }
 }