Switch hashCode to not use toStringTree()

Node doesn't currently have a hashCode method so this will just fallback to the built in Object hashCode. But since this is only used in one HashMap (throwsDescriptions in JSDocInfo, which is likely to only have a few entries) I don't think having a highly accurate hashCode method is very important here.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152752320

Author: tbreisacher

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

@@ -111,7 +111,7 @@ public boolean equals(Object other) {
 
   @Override
   public int hashCode() {
-    return root.toStringTree().hashCode();
+    return root.hashCode();
   }
 
   /**