allow optional parameters for types using fromJson()

json_serializable/lib/src/type_helpers/json_helper.dart

@@ -33,7 +33,7 @@ class JsonHelper extends TypeHelper {
     var fromJsonCtor =
         classElement.constructors.firstWhere((ce) => ce.name == 'fromJson');
     // TODO: should verify that this type is a valid JSON type...but for now...
-    var asCastType = fromJsonCtor.parameters.single.type;
+    var asCastType = fromJsonCtor.parameters.first.type;
 
     var asCast = '';
     if (!asCastType.isDynamic && !asCastType.isObject) {

json_serializable/test/json_serializable_test.dart

@@ -188,6 +188,13 @@ abstract class _$OrderSerializerMixin {
       });
     }
 
+
+    test('class with fromJson() constructor with optional parameters', () async {
+      var output = await runForElementNamed('FromJsonOptionalParameters');
+
+      expect(output, contains('new ChildWithFromJson.fromJson'));
+    });
+
     test('class with child json-able object', () async {
       var output = await runForElementNamed('ParentObject');
 
@@ -321,6 +328,17 @@ class FinalFields {
   FinalFields(this.a);
 }
 
+@JsonSerializable()
+class FromJsonOptionalParameters {
+  final ChildWithFromJson child;
+
+  FromJsonOptionalParameters(this.child);
+}
+
+class ChildWithFromJson {
+  ChildWithFromJson.fromJson(json, {initValue: false}) {}
+}
+
 @JsonSerializable()
 class ParentObject {
   int number;