Fix memory leak in SingletonImmutableBiMap which would appear in transpiled J2ObjC code.

The @RetainedWith annotation can not be used on both sides of the retain-cycle. The RegularImmutableBiMap does not have this problem, because it uses Inverse inner class. This CL applies similar trick in SingletonImmutableBiMap, although without additional inner class.

RELNOTES=Update SingletonImmutableBiMap to avoid retain-cycle in transpiled Obj-C code.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=317856944

Author: micapolos-google

cycle_whitelist.txt

@@ -13,8 +13,6 @@ NAMESPACE junit.framework
 NAMESPACE org.junit
 
 # ***** REAL CYCLES *****
-# Inverses (currently not solvable by weakening a reference)
-FIELD com.google.common.base.Converter.reverse
 # Cycle exists until future completes
 FIELD com.google.common.util.concurrent.AbstractFuture.Listener.executor com.google.common.util.concurrent.ExecutionSequencer.TaskNonReentrantExecutor
 
@@ -23,6 +21,7 @@ FIELD com.google.common.util.concurrent.AbstractFuture.Listener.executor com.goo
 # The Runnable type is so generic that it produces too many false positives.
 TYPE java.lang.Runnable
 
+FIELD com.google.common.base.Converter.reverse
 FIELD com.google.common.collect.AbstractBiMap.EntrySet.iterator.$.entry com.google.common.collect.AbstractBiMap.EntrySet.iterator.$.next.$
 FIELD com.google.common.collect.AbstractMapBasedMultimap.map
 FIELD com.google.common.collect.AbstractMultimap.asMap com.google.common.collect.AbstractMapBasedMultimap.NavigableAsMap
@@ -36,6 +35,7 @@ FIELD com.google.common.collect.ImmutableRangeSet.ranges
 FIELD com.google.common.collect.ImmutableSet.asList
 FIELD com.google.common.collect.Maps.FilteredMapValues.unfiltered
 FIELD com.google.common.collect.Sets.SubSet.inputSet
+FIELD com.google.common.collect.SingletonImmutableBiMap.inverse
 FIELD com.google.common.collect.TreeTraverser.PostOrderNode.childIterator
 FIELD com.google.common.collect.TreeTraverser.PreOrderIterator.stack
 FIELD com.google.common.util.concurrent.AbstractFuture.Listener.executor com.google.common.util.concurrent.MoreExecutors.rejectionPropagatingExecutor.$

guava/src/com/google/common/collect/SingletonImmutableBiMap.java

@@ -42,6 +42,7 @@ final class SingletonImmutableBiMap<K, V> extends ImmutableBiMap<K, V> {
     checkEntryNotNull(singleKey, singleValue);
     this.singleKey = singleKey;
     this.singleValue = singleValue;
+    this.inverse = null;
   }
 
   private SingletonImmutableBiMap(K singleKey, V singleValue, ImmutableBiMap<V, K> inverse) {
@@ -90,16 +91,21 @@ ImmutableSet<K> createKeySet() {
     return ImmutableSet.of(singleKey);
   }
 
-  @LazyInit @RetainedWith transient ImmutableBiMap<V, K> inverse;
+  private final transient @Nullable ImmutableBiMap<V, K> inverse;
+  @LazyInit @RetainedWith private transient @Nullable ImmutableBiMap<V, K> lazyInverse;
 
   @Override
   public ImmutableBiMap<V, K> inverse() {
-    // racy single-check idiom
-    ImmutableBiMap<V, K> result = inverse;
-    if (result == null) {
-      return inverse = new SingletonImmutableBiMap<>(singleValue, singleKey, this);
+    if (inverse != null) {
+      return inverse;
     } else {
-      return result;
+      // racy single-check idiom
+      ImmutableBiMap<V, K> result = lazyInverse;
+      if (result == null) {
+        return lazyInverse = new SingletonImmutableBiMap<>(singleValue, singleKey, this);
+      } else {
+        return result;
+      }
     }
   }
 }