Fix various bits in testing code to comply with new warnings (#3307)

* Fix various bits in testing code to comply with new warnings

* try-again

Author: srawlins

testing/test_package/lib/fake.dart

@@ -465,8 +465,7 @@ abstract class ImplementsFutureVoid implements Future<void> {}
 
 /// This class takes a type, and it might be void.
 class ATypeTakingClass<T> {
-  // ignore: missing_return
-  T? aMethodMaybeReturningVoid() {}
+  T? aMethodMaybeReturningVoid() => null;
 }
 
 class ABaseClass {}
@@ -1041,7 +1040,7 @@ class OperatorReferenceClass {
   OperatorReferenceClass();
 
   @override
-  bool operator ==(dynamic other) {
+  bool operator ==(Object other) {
     return false;
   }
 }
@@ -1283,7 +1282,7 @@ class FactoryConstructorThings {
 
 DTypeParam?
     aTopLevelTypeParameterFunction<DTypeParam extends TypeParameterThings>(
-        DTypeParam typedParam) {}
+        DTypeParam typedParam) => typedParam;
 
 abstract class TypeParameterThings<ATypeParam,
     BTypeParam extends FactoryConstructorThings> {

testing/test_package/lib/features/generalized_typedefs.dart

@@ -24,7 +24,7 @@ T0 a;
 class C2 {
   T0 b;
   T1? c;
-  T2? d(T3 e, T4 f) {}
+  T2? d(T3 e, T4 f) => null;
 }
 
 class C1<T extends T3> {