Log warnings for users who send Guava types over GWT-RPC.

I've left this change out of the Android branch. For starters, it's not necessary there, since we don't support GWT. But normally we'd include it anyway to minimize diffs. However, doing so increases the number of classes retained after running Proguard (thanks, I assume, to the Logger field). There are other ways of getting around this, but for a temporary feature, the diffs seem tolerable.

RELNOTES=***Warning to users of GWT-RPC:*** The first Guava release of 2020 will [remove support for GWT-serializing Guava types](https://groups.google.com/d/msg/guava-announce/zHZTFg7YF3o/rQNnwdHeEwAJ). To test ahead of time with GWT-RPC support for Guava types disabled, set the `guava.gwt.emergency_reenable_rpc` system property to `false`.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=273803350

guava/src/com/google/common/base/Platform.java

@@ -111,5 +111,12 @@ static void checkGwtRpcEnabled() {
               "https://stackoverflow.com/q/5189914/28465",
               "https://groups.google.com/d/msg/guava-announce/zHZTFg7YF3o/rQNnwdHeEwAJ"));
     }
+    logger.log(
+        java.util.logging.Level.WARNING,
+        "In January 2020, we will remove GWT-RPC support for Guava types. You are seeing this"
+            + " warning because you are sending a Guava type over GWT-RPC, which will break. You"
+            + " can identify which type by looking at the class name in the attached stack trace.",
+        new Throwable());
+
   }
 }

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

@@ -32,6 +32,9 @@
  */
 @GwtCompatible(emulated = true)
 final class Platform {
+  private static final java.util.logging.Logger logger =
+      java.util.logging.Logger.getLogger(Platform.class.getName());
+
   /** Returns the platform preferred implementation of a map based on a hash table. */
   static <K, V> Map<K, V> newHashMapWithExpectedSize(int expectedSize) {
     return Maps.newHashMapWithExpectedSize(expectedSize);
@@ -125,6 +128,13 @@ static void checkGwtRpcEnabled() {
               "https://stackoverflow.com/q/5189914/28465",
               "https://groups.google.com/d/msg/guava-announce/zHZTFg7YF3o/rQNnwdHeEwAJ"));
     }
+    logger.log(
+        java.util.logging.Level.WARNING,
+        "In January 2020, we will remove GWT-RPC support for Guava types. You are seeing this"
+            + " warning because you are sending a Guava type over GWT-RPC, which will break. You"
+            + " can identify which type by looking at the class name in the attached stack trace.",
+        new Throwable());
+
   }
 
   private Platform() {}

guava/src/com/google/common/primitives/Platform.java

@@ -22,6 +22,9 @@
 /** Methods factored out so that they can be emulated differently in GWT. */
 @GwtCompatible(emulated = true)
 final class Platform {
+  private static final java.util.logging.Logger logger =
+      java.util.logging.Logger.getLogger(Platform.class.getName());
+
   private static final String GWT_RPC_PROPERTY_NAME = "guava.gwt.emergency_reenable_rpc";
 
   static void checkGwtRpcEnabled() {
@@ -35,6 +38,13 @@ static void checkGwtRpcEnabled() {
               "https://stackoverflow.com/q/5189914/28465",
               "https://groups.google.com/d/msg/guava-announce/zHZTFg7YF3o/rQNnwdHeEwAJ"));
     }
+    logger.log(
+        java.util.logging.Level.WARNING,
+        "In January 2020, we will remove GWT-RPC support for Guava types. You are seeing this"
+            + " warning because you are sending a Guava type over GWT-RPC, which will break. You"
+            + " can identify which type by looking at the class name in the attached stack trace.",
+        new Throwable());
+
   }
 
   private Platform() {}