Add support for the postfix ! operator (#325)

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 * Add `Expression.spread` for the spread operator `...`.
 * Add support 'late' field modifier.
+* Add support for `Expression.nullChecked` to add a null assertion operator.
 
 ## 4.0.0
 

lib/src/specs/expression.dart

@@ -277,6 +277,17 @@ abstract class Expression implements Spec {
         addSpace: false,
       );
 
+  /// Applies the null check operator on this expression, returning `this` `!`.
+  ///
+  /// Please note that this is only valid when emitting code with the null
+  /// safety syntax enabled.
+  Expression get nullChecked => BinaryExpression._(
+        this,
+        const LiteralExpression._('!'),
+        '',
+        addSpace: false,
+      );
+
   /// This expression preceded by `return`.
   Expression get returned => BinaryExpression._(
         const LiteralExpression._('return'),

test/specs/code/expression_test.dart

@@ -377,6 +377,10 @@ void main() {
     );
   });
 
+  test('should emit a null check', () {
+    expect(refer('foo').nullChecked, equalsDart('foo!'));
+  });
+
   test('should emit an if null index operator set', () {
     expect(
       refer('bar')