code: private CollectTweaksResult collectTweaks(Node root) {
      CollectTweaks pass = new CollectTweaks();
      NodeTraversal.traverse(compiler, root, pass);
  
      Map<String, TweakInfo> tweakInfos = pass.allTweaks;
      for (TweakInfo tweakInfo : tweakInfos.values()) {
        tweakInfo.emitAllWarnings();
      }
      return new CollectTweaksResult(tweakInfos, pass.getOverridesCalls);
    }
docs: Finds all calls to goog.tweak functions and emits warnings/errors if any
  of the calls have issues.
  @return A map of {@link TweakInfo} structures, keyed by tweak ID.