gracefully handle unsupported phone auth methods

Author: cbenhagen

packages/firebase_auth/firebase_auth/ios/firebase_auth/Sources/firebase_auth/FLTFirebaseAuthPlugin.m

@@ -850,12 +850,16 @@ - (nonnull ASPresentationAnchor)presentationAnchorForAuthorizationController:
 #endif
 }
 
-#if TARGET_OS_IPHONE
-
 - (void)enrollPhoneApp:(nonnull AuthPigeonFirebaseApp *)app
              assertion:(nonnull PigeonPhoneMultiFactorAssertion *)assertion
            displayName:(nullable NSString *)displayName
             completion:(nonnull void (^)(FlutterError *_Nullable))completion {
+#if TARGET_OS_OSX
+  completion([FlutterError errorWithCode:@"unsupported-platform"
+                                 message:@"Phone authentication is not supported on macOS"
+                                 details:nil]);
+#else
+
   FIRMultiFactor *multiFactor = [self getAppMultiFactorFromPigeon:app];
 
   FIRPhoneAuthCredential *credential =
@@ -877,9 +881,8 @@ - (void)enrollPhoneApp:(nonnull AuthPigeonFirebaseApp *)app
                                                            details:nil]);
                           }
                         }];
-}
-
 #endif
+}
 
 - (void)getEnrolledFactorsApp:(nonnull AuthPigeonFirebaseApp *)app
                    completion:(nonnull void (^)(NSArray<PigeonMultiFactorInfo *> *_Nullable,
@@ -959,13 +962,16 @@ - (void)enrollTotpApp:(nonnull AuthPigeonFirebaseApp *)app
                         }];
 }
 
-#if TARGET_OS_IPHONE
-
 - (void)resolveSignInResolverId:(nonnull NSString *)resolverId
                       assertion:(nullable PigeonPhoneMultiFactorAssertion *)assertion
                 totpAssertionId:(nullable NSString *)totpAssertionId
                      completion:(nonnull void (^)(PigeonUserCredential *_Nullable,
                                                   FlutterError *_Nullable))completion {
+#if TARGET_OS_OSX
+  completion(nil, [FlutterError errorWithCode:@"unsupported-platform"
+                                     message:@"Phone authentication is not supported on macOS"
+                                     details:nil]);
+#else
   FIRMultiFactorResolver *resolver = _multiFactorResolverMap[resolverId];
 
   FIRMultiFactorAssertion *multiFactorAssertion;
@@ -999,9 +1005,8 @@ - (void)resolveSignInResolverId:(nonnull NSString *)resolverId
                                                               details:nil]);
                         }
                       }];
-}
-
 #endif
+}
 
 - (void)generateSecretSessionId:(nonnull NSString *)sessionId
                      completion:(nonnull void (^)(PigeonTotpSecret *_Nullable,