[pigeon] Validates support for nested collections with generic types. (flutter#7614)

Validates support for nested collections with generic types.

Really just adds tests to ensure behavior. 

small step toward flutter#116117

Author: tarrinneal

packages/pigeon/pigeons/core_tests.dart

@@ -52,13 +52,16 @@ class AllTypes {
     required this.enumList,
     required this.objectList,
     required this.listList,
+    required this.mapList,
 
     // Maps
     required this.map,
     required this.stringMap,
     required this.intMap,
     required this.enumMap,
     required this.objectMap,
+    required this.listMap,
+    required this.mapMap,
   });
 
   bool aBool;
@@ -84,6 +87,7 @@ class AllTypes {
   List<AnEnum> enumList;
   List<Object> objectList;
   List<List<Object?>> listList;
+  List<Map<Object?, Object?>> mapList;
 
   // Maps
   // ignore: strict_raw_type, always_specify_types
@@ -92,6 +96,8 @@ class AllTypes {
   Map<int, int> intMap;
   Map<AnEnum, AnEnum> enumMap;
   Map<Object, Object> objectMap;
+  Map<int, List<Object?>> listMap;
+  Map<int, Map<Object?, Object?>> mapMap;
 }
 
 /// A class containing all supported nullable types.
@@ -123,6 +129,7 @@ class AllNullableTypes {
     this.enumList,
     this.objectList,
     this.listList,
+    this.mapList,
     this.recursiveClassList,
 
     // Maps
@@ -131,6 +138,8 @@ class AllNullableTypes {
     this.intMap,
     this.enumMap,
     this.objectMap,
+    this.listMap,
+    this.mapMap,
     this.recursiveClassMap,
   );
 
@@ -158,6 +167,7 @@ class AllNullableTypes {
   List<AnEnum?>? enumList;
   List<Object?>? objectList;
   List<List<Object?>?>? listList;
+  List<Map<Object?, Object?>?>? mapList;
   List<AllNullableTypes?>? recursiveClassList;
 
   // Maps
@@ -167,6 +177,8 @@ class AllNullableTypes {
   Map<int?, int?>? intMap;
   Map<AnEnum?, AnEnum?>? enumMap;
   Map<Object?, Object?>? objectMap;
+  Map<int?, List<Object?>?>? listMap;
+  Map<int?, Map<Object?, Object?>?>? mapMap;
   Map<int?, AllNullableTypes?>? recursiveClassMap;
 }
 
@@ -199,13 +211,16 @@ class AllNullableTypesWithoutRecursion {
     this.enumList,
     this.objectList,
     this.listList,
+    this.mapList,
 
     // Maps
     this.map,
     this.stringMap,
     this.intMap,
     this.enumMap,
     this.objectMap,
+    this.listMap,
+    this.mapMap,
   );
 
   bool? aNullableBool;
@@ -231,6 +246,7 @@ class AllNullableTypesWithoutRecursion {
   List<AnEnum?>? enumList;
   List<Object?>? objectList;
   List<List<Object?>?>? listList;
+  List<Map<Object?, Object?>?>? mapList;
 
   // Maps
   // ignore: strict_raw_type, always_specify_types
@@ -239,6 +255,8 @@ class AllNullableTypesWithoutRecursion {
   Map<int?, int?>? intMap;
   Map<AnEnum?, AnEnum?>? enumMap;
   Map<Object?, Object?>? objectMap;
+  Map<int?, List<Object?>?>? listMap;
+  Map<int?, Map<Object?, Object?>?>? mapMap;
 }
 
 /// A class for testing nested class handling.