Refactor the Event class; always construct through static methods.

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MOS_MIGRATED_REVID=120418505

src/main/java/com/google/devtools/build/lib/actions/ActionCacheChecker.java

diff --git a/src/main/java/com/google/devtools/build/lib/actions/ActionCacheChecker.java b/src/main/java/com/google/devtools/build/lib/actions/ActionCacheChecker.java
index 72d5756..4fd06a9 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/ActionCacheChecker.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/ActionCacheChecker.java
@@ -327,7 +327,7 @@
   private static void reportRebuild(@Nullable EventHandler handler, Action action, String message) {
     // For MiddlemanAction, do not report rebuild.
     if (handler != null && !action.getActionType().isMiddleman()) {
-      handler.handle(new Event(
+      handler.handle(Event.of(
           EventKind.DEPCHECKER, null, "Executing " + action.prettyPrint() + ": " + message + "."));
     }
   }