Null check operator used on a null value

[HXiaoMing]

test case:

import 'package:dart_eval/dart_eval.dart';
import 'package:test/scaffolding.dart';

void main() {

  test('nullable static value', () {
    final program = '''
            class TestA {
              static int? value;
            }      

            int? main() {
              TestA.value = 22;
              return TestA.value;
            }
      ''';

    final result = eval(program, function: 'main');
    print('result $result');
  });

}

Error: package:dart_eval/src/eval/compiler/reference.dart 101:79 IdentifierReference.setValue package:dart_eval/src/eval/compiler/expression/assignment.dart 18:14 compileAssignmentExpression package:dart_eval/src/eval/compiler/expression/expression.dart 34:12 compileExpression package:dart_eval/src/eval/compiler/expression/expression.dart 106:12 compileExpressionAndDiscardResult package:dart_eval/src/eval/compiler/statement/statement.dart 26:15 compileStatement package:dart_eval/src/eval/compiler/statement/block.dart 19:20 compileBlock package:dart_eval/src/eval/compiler/declaration/function.dart 84:14 compileFunctionDeclaration package:dart_eval/src/eval/compiler/declaration/declaration.dart 24:5 compileDeclaration package:dart_eval/src/eval/compiler/compiler.dart 493:11 Compiler.compileSources.. dart:collection _LinkedHashMapMixin.forEach package:dart_eval/src/eval/compiler/compiler.dart 482:15 Compiler.compileSources. dart:collection _LinkedHashMapMixin.forEach package:dart_eval/src/eval/compiler/compiler.dart 478:32 Compiler.compileSources package:dart_eval/src/eval/compiler/compiler.dart 164:12 Compiler.compile package:dart_eval/src/eval/eval.dart 40:28 eval test/custom/change_static_value.dart 18:20 main.

Null check operator used on a null value

[ethanblake4]

Fixed in v0.7.9