Fix a minor code-health-ish TODO.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=178634431

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

@@ -2343,7 +2343,7 @@ public final Node useSourceInfoIfMissingFromForTree(Node other) {
    * #getDeclaredTypeExpression()} which returns the syntactically specified type.
    */
   @Nullable
-  public JSType getJSType() {  // TODO(johnlenz): make this final
+  public final JSType getJSType() {
     return typei instanceof JSType ? (JSType) typei : null;
   }