Recommend 4.3.0 version of google-services in READMEs/examples

[collinjackson]

Per discussion flutter/flutter#28770 (comment)

We can prevent these deprecation warnings by updating to a newer version of google-services gradle plugin.

flutter run                                                                                                     11:32:05
Launching lib/main.dart on Pixel 2 in debug mode...
Initializing gradle...                                              1.9s
Resolving dependencies...                                           5.2s
registerResGeneratingTask is deprecated, use registerGeneratedResFolders(FileCollection)
registerResGeneratingTask is deprecated, use registerGeneratedResFolders(FileCollection)
registerResGeneratingTask is deprecated, use registerGeneratedResFolders(FileCollection)
Note: /Users/deg/.pub-cache/hosted/pub.dartlang.org/firebase_core-0.4.0+6/android/src/main/java/io/flutter/plugins/firebase/core/FirebaseCorePlugin.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.                      
Running Gradle task 'assembleDebug'...                                  
Running Gradle task 'assembleDebug'... Done                        45.0s
Built build/app/outputs/apk/debug/app-debug.apk.
...

This change will cause warnings to be emitted to the console the first time building after upgrading an app's google-services.

ERROR: [TAG] Failed to resolve variable '${junit.version}'              
ERROR: [TAG] Failed to resolve variable '${animal.sniffer.version}'     

Subsequent builds will not have the error.

Fixes fluttere/flutter#28982

[kroikie]

LGTM

[collinjackson]

Landing this on red because the analyzer failures are a known issue (see #1882) and iOS build timeouts cannot be caused by Android-only changes.

[repentsinner]

Thanks for this @collinjackson!

Is this type of thing something that flutter doctor should be checking for?

Coming up to speed with Flutter feels like I spend the same amount of effort hunting down Magic Boilerplate issues as I do figuring out what's wrong with my own code.

[cremor]

@collinjackson Why was this added? Is it recommended to also depend on firebase_analytics when you use firebase_messaging?