Add MixedMutabilityReturnType suppression

Change-Id: Ia703e2092b669a593405a45713097e1de275c13e
diff --git a/src/main/java/com/android/tools/r8/shaking/IfRuleEvaluatorFactory.java b/src/main/java/com/android/tools/r8/shaking/IfRuleEvaluatorFactory.java
index d22eb63..04026fd 100644
--- a/src/main/java/com/android/tools/r8/shaking/IfRuleEvaluatorFactory.java
+++ b/src/main/java/com/android/tools/r8/shaking/IfRuleEvaluatorFactory.java
@@ -40,6 +40,7 @@
     this.tasks = new TaskCollection<>(appView.options(), executorService);
   }
 
+  @SuppressWarnings("MixedMutabilityReturnType")
   private static Map<Wrapper<ProguardIfRule>, Set<ProguardIfRule>> createActiveIfRules(
       Set<ProguardIfRule> ifRules) {
     if (ifRules == null || ifRules.isEmpty()) {