Bring back previously deleted SerializedForms so that old data can st…

…ill be read.

RELNOTES=`collect`: Data that was previously serialized, and that originated from ImmutableMap.keySet() or .values(), is now deserializable again.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=326086350

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

@@ -19,6 +19,8 @@
 import static com.google.common.base.Preconditions.checkNotNull;
 
 import com.google.common.annotations.GwtCompatible;
+import com.google.common.annotations.GwtIncompatible;
+import java.io.Serializable;
 import java.util.Spliterator;
 import java.util.function.Consumer;
 import org.checkerframework.checker.nullness.qual.Nullable;
@@ -72,4 +74,21 @@ public void forEach(Consumer<? super K> action) {
   boolean isPartialView() {
     return true;
   }
+
+  // No longer used for new writes, but kept so that old data can still be read.
+  @GwtIncompatible // serialization
+  @SuppressWarnings("unused")
+  private static class KeySetSerializedForm<K> implements Serializable {
+    final ImmutableMap<K, ?> map;
+
+    KeySetSerializedForm(ImmutableMap<K, ?> map) {
+      this.map = map;
+    }
+
+    Object readResolve() {
+      return map.keySet();
+    }
+
+    private static final long serialVersionUID = 0;
+  }
 }

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

@@ -20,6 +20,7 @@
 
 import com.google.common.annotations.GwtCompatible;
 import com.google.common.annotations.GwtIncompatible;
+import java.io.Serializable;
 import java.util.Map.Entry;
 import java.util.Spliterator;
 import java.util.function.Consumer;
@@ -98,4 +99,21 @@ public void forEach(Consumer<? super V> action) {
     checkNotNull(action);
     map.forEach((k, v) -> action.accept(v));
   }
+
+  // No longer used for new writes, but kept so that old data can still be read.
+  @GwtIncompatible // serialization
+  @SuppressWarnings("unused")
+  private static class SerializedForm<V> implements Serializable {
+    final ImmutableMap<?, V> map;
+
+    SerializedForm(ImmutableMap<?, V> map) {
+      this.map = map;
+    }
+
+    Object readResolve() {
+      return map.values();
+    }
+
+    private static final long serialVersionUID = 0;
+  }
 }

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

@@ -22,9 +22,11 @@
 import static com.google.common.collect.ImmutableMapEntry.createEntryArray;
 
 import com.google.common.annotations.GwtCompatible;
+import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.collect.ImmutableMapEntry.NonTerminalImmutableMapEntry;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
+import java.io.Serializable;
 import java.util.function.BiConsumer;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
@@ -231,6 +233,23 @@ boolean isPartialView() {
     public int size() {
       return map.size();
     }
+
+    // No longer used for new writes, but kept so that old data can still be read.
+    @GwtIncompatible // serialization
+    @SuppressWarnings("unused")
+    private static class SerializedForm<K> implements Serializable {
+      final ImmutableMap<K, ?> map;
+
+      SerializedForm(ImmutableMap<K, ?> map) {
+        this.map = map;
+      }
+
+      Object readResolve() {
+        return map.keySet();
+      }
+
+      private static final long serialVersionUID = 0;
+    }
   }
 
   @Override
@@ -260,6 +279,23 @@ public int size() {
     boolean isPartialView() {
       return true;
     }
+
+    // No longer used for new writes, but kept so that old data can still be read.
+    @GwtIncompatible // serialization
+    @SuppressWarnings("unused")
+    private static class SerializedForm<V> implements Serializable {
+      final ImmutableMap<?, V> map;
+
+      SerializedForm(ImmutableMap<?, V> map) {
+        this.map = map;
+      }
+
+      Object readResolve() {
+        return map.values();
+      }
+
+      private static final long serialVersionUID = 0;
+    }
   }
 
   // This class is never actually serialized directly, but we have to make the