iOS compilation fails in Flutter app for XCode 9.2

[camillo777]

• Xcode version: Version 9.2 (9C40b)
• Firebase SDK version: 7.6.1
• Firebase Component: _____ (Auth, Core, Database, Firestore, Messaging, Storage, etc)
• Component version: _____
• Mac OSX Sierra 10.12.6 (16G2128)
• Flutter flutter version v1.9.1+hotfix.5

Describe the problem:
I am trying to build my really Flutter simple app for iOS (Flutter init app).
Android is ok.
I cannot update OSX so I am stuck with XCode 9.2.
On iPhone6 and Simulator iPhoneX build fails with the attached error message.

Here it says it is a Firebase iOS SDK problem:
flutter/flutter#40527

Installing new DeviceSupport files did not help as well:

$ cd /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/
10.0/          10.3/          11.2 (15C107)/ 13.2/          8.2/           9.0/           9.3/           
10.1/          11.0/          12.4 (16G73)/  8.0/           8.3/           9.1/           
10.2/          11.1/          13.1/          8.1/           8.4/           9.2/     

Visual Studio Code errors:

Launching lib/main.dart on iPhone X in debug mode...
Xcode build done.                                            3.8s
Failed to build iOS app
Error output from Xcode build:
↳
    2019-10-19 21:28:06.684 xcodebuild[21473:184514]  DVTAssertions: Warning in /Library/Caches/com.apple.xbs/Sources/IDEXcode3ProjectSupport/IDEXcode3ProjectSupport-13756/Xcode3Core/LegacyProjects/Frameworks/DevToolsCore/DevToolsCore/BuildSystem/Runtime/PBXTargetBuildContext.mm:757
    Details:  unexpected successful exit code from cancelled command <C0003:'CpResource roots.pem':P8>
    Object:   <PBXTargetBuildContext: 0x7fb83187dde0>
    Method:   -createCommandInvocationRecordFromInvocation:
    Thread:   <NSThread: 0x7fb833a35750>{number = 17, name = (null)}
    Please file a bug at http://bugreport.apple.com with this warning message and any useful information you can provide.
    ** BUILD FAILED **
Xcode's output:
↳
    === BUILD TARGET leveldb-library OF PROJECT Pods WITH CONFIGURATION Debug ===
    /Users/camillo/MyCamillo/Development/flutter/camillo_flutter/ios/Pods/GoogleDataTransport/GoogleDataTransport/GDTLibrary/GDTStorage.m:118:41: error: no known class method for selector 'archivedDataWithRootObject:requiringSecureCoding:error:'
            NSData *data = [NSKeyedArchiver archivedDataWithRootObject:self
                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /Users/camillo/MyCamillo/Development/flutter/camillo_flutter/ios/Pods/GoogleDataTransport/GoogleDataTransport/GDTLibrary/GDTStorage.m:215:24: error: no known class method for selector 'unarchivedObjectOfClass:fromData:error:'
        [NSKeyedUnarchiver unarchivedObjectOfClass:[GDTStorage class] fromData:data error:nil];
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /Users/camillo/MyCamillo/Development/flutter/camillo_flutter/ios/Pods/GoogleDataTransport/GoogleDataTransport/GDTLibrary/GDTStorage.m:227:39: error: no known class method for selector 'archivedDataWithRootObject:requiringSecureCoding:error:'
          NSData *data = [NSKeyedArchiver archivedDataWithRootObject:self
                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /Users/camillo/MyCamillo/Development/flutter/camillo_flutter/ios/Pods/GoogleDataTransport/GoogleDataTransport/GDTLibrary/GDTStorage.m:255:37: error: no known class method for selector 'archivedDataWithRootObject:requiringSecureCoding:error:'
        NSData *data = [NSKeyedArchiver archivedDataWithRootObject:self
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    4 errors generated.
Could not build the application for the simulator.
Error launching application on iPhone X.

[paulb777]

@camillo777 Xcode 10.1 is currently the earliest supported Xcode version.

Apple requires at least Xcode 10 for app store submissions and will require Xcode 11 starting in April 2020.

A workaround for you could be to comment out/or delete the erroring code so the old API in the else statement below is always executed.

However, no guarantees that there might not be other issues using Xcode 9.x