Add @CheckReturnValue to help prevent useless (and potentially expensive) toStringTree calls

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152753986

Author: tbreisacher

src/com/google/javascript/rhino/Node.java

@@ -54,6 +54,7 @@
 import java.util.Iterator;
 import java.util.NoSuchElementException;
 import java.util.Set;
+import javax.annotation.CheckReturnValue;
 import javax.annotation.Nullable;
 
 /**
@@ -1203,7 +1204,7 @@ private void toString(
     }
   }
 
-
+  @CheckReturnValue
   public String toStringTree() {
     return toStringTreeImpl();
   }