Repository: flutter
Issue: Google Sign In / Android - missing Guava Proguard rules
Labels: c: crash,platform-android,t: gradle,p: google_sign_in,package,a: release,P3,team-android,triaged-android

When trying to build release, I am getting proguard errors:

```
Note: the configuration refers to the unknown class 'com.google.android.gms.common.api.internal.BasePendingResult$ReleasableResultGuardian'
Warning: com.google.common.util.concurrent.FuturesGetChecked$GetCheckedTypeValidatorHolder$ClassValueValidator$1: can't find superclass or interface java.lang.ClassValue
Warning: com.google.common.base.AbstractIterator: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Converter: can't find referenced class com.google.errorprone.annotations.concurrent.LazyInit
Warning: com.google.common.base.Converter: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Converter: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Converter: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Converter: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Function: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Joiner: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Joiner: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Joiner: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Joiner: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Joiner: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Joiner: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
Warning: com.google.common.base.Joiner: can't find referenced class com.google.errorprone.annotations.CanIgnoreReturnValue
```

```
Warning: there were 696 unresolved references to classes or interfaces. 
         You may need to add missing library jars or update their versions.
         If your code works fine without the missing classes, you can suppress
         the warnings with '-dontwarn' options.                         
         (http://proguard.sourceforge.net/manual/troubleshooting.html#unresolvedclass)
                                                                        
FAILURE: Build failed with an exception.                                

```

I need to create proguard file and add Guava 20 rules:

```
-dontwarn com.google.common.base.**
-keep class com.google.common.base.** {*;}
-dontwarn com.google.errorprone.annotations.**
-keep class com.google.errorprone.annotations.** {*;}
-dontwarn   com.google.j2objc.annotations.**
-keep class com.google.j2objc.annotations.** { *; }
-dontwarn   java.lang.ClassValue
-keep class java.lang.ClassValue { *; }
-dontwarn   org.codehaus.mojo.animal_sniffer.IgnoreJRERequirement
-keep class org.codehaus.mojo.animal_sniffer.IgnoreJRERequirement { *; }
```
to build release with proguard enabled (which is default currently). You need to fix plugin sources and add proguard rules there.

BTW. You actually should not use Guava, as most Java 8 features is available now by desugaring: https://developer.android.com/studio/write/java8-support or simply use Kotiln.