Avoid use of NonConstantValue in SSA

pkg/compiler/lib/src/constants/values.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.
 
 library dart2js.constants.values;
 
 import '../common.dart';
 import '../core_types.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart'
@@ skipping 141 matching lines @@
   DartString toDartString();
 
   /// This value in Dart syntax.
   String toDartText() => primitiveValue.toString();
 }
 
 class NullConstantValue extends PrimitiveConstantValue {
   /** The value a Dart null is compiled to in JavaScript. */
   static const String JsNull = "null";
 
-  factory NullConstantValue() => const NullConstantValue._internal();
+  const factory NullConstantValue() = NullConstantValue._internal;

[Siggi Cherem (dart-lang), 2016/04/29 20:26:40]

I didn't know we were allowed to declare a factory constructor as const

[Johnni Winther, 2016/04/30 09:06:02]

When they are redirecting we can (I changed that too).

 
   const NullConstantValue._internal();
 
   bool get isNull => true;
 
   get primitiveValue => null;
 
   DartType getType(CoreTypes types) => types.nullType;
 
   // The magic constant has no meaning. It is just a random value.
@@ skipping 597 matching lines @@
 
   @override
   DartType getType(CoreTypes types) => const DynamicType();
 
   @override
   String toStructuredText() => 'NonConstant';
 
   @override
   String toDartText() => '>>non-constant<<';
 }