Added type safety to equal validator, along with a unit test.

Author: awhitford

lib/src/form_builder_validators.dart

@@ -39,9 +39,9 @@ class FormBuilderValidators {
 
   /// [FormFieldValidator] that requires the field's value be true.
   /// Commonly used for required checkboxes.
-  static FormFieldValidator equal(
+  static FormFieldValidator<T> equal<T>(
     BuildContext context,
-    dynamic value, {
+    T value, {
     String errorText,
   }) =>
       (valueCandidate) => valueCandidate != value

test/form_builder_validators_test.dart

@@ -44,6 +44,17 @@ void main() {
             expect(validator([]), isNotNull);
           }));
 
+  testWidgets(
+      'FormBuilderValidators.equal',
+      (WidgetTester tester) => testValidations(tester, (context) {
+            final validator = FormBuilderValidators.equal(context, true);
+            // Pass
+            expect(validator(true), isNull);
+            // Fail
+            expect(validator(null), isNotNull);
+            expect(validator(false), isNotNull);
+          }));
+
   testWidgets(
       'FormBuilderValidators.maxLength',
       (WidgetTester tester) => testValidations(tester, (context) {