public synchronized void intern(CstCallSiteRef cstRef) {
    if (cstRef == null) {
      throw new NullPointerException("cstRef");
    }

    throwIfPrepared();

    CallSiteIdItem result = callSiteIds.get(cstRef);
    if (result == null) {
      result = new CallSiteIdItem(cstRef);
      callSiteIds.put(cstRef, result);
    }
  }
Interns a call site into this instance.

<p>This method is synchronized as it is called during class file translation which runs
concurrently on a per class basis.

@param cstRef
android/android_framework/dex-maker/src/main/java/com/android/dx/dex/file/CallSiteIdsSection.java