anchor.diff:        addCompileTimeError(
             token.charOffset, "const field must have initializer.");
         // Creating a null value to prevent the Dart VM from crashing.
  -      push(new NullLiteral()..fileOffset = token.charOffset);
  +      push(astFactory.nullLiteral(token));
       } else {
         push(NullValue.FieldInitializer);
       }
  
positive.code:   @override
    void handleLiteralNull(Token token) {
      debugEvent("LiteralNull");
      push(new NullLiteral()..fileOffset = token.charOffset);
    }
  
    @override
  
negative.code:         bool hasMore = formalBuilders.moveNext();
          assert(hasMore);
          VariableDeclaration realParameter = formalBuilders.current.target;
          Expression initializer =
              parameter.initializer ?? astFactory.nullLiteral(null);
          realParameter.initializer = initializer..parent = realParameter;
        }
      }
  