code: private void visitFallbackFunctionCall(NodeTraversal t, Node call) {
      // Check to make sure the function call looks like:
      // goog.getMsgWithFallback(MSG_1, MSG_2);
      if (call.getChildCount() != 3
          || !call.getSecondChild().isName()
          || !call.getLastChild().isName()) {
        compiler.report(JSError.make(call, BAD_FALLBACK_SYNTAX));
        return;
      }
  
      Node firstArg = call.getSecondChild();
      String name = firstArg.getOriginalName();
      if (name == null) {
        name = firstArg.getString();
      }
      JsMessage firstMessage = getTrackedMessage(t, name);
      if (firstMessage == null) {
        compiler.report(JSError.make(firstArg, FALLBACK_ARG_ERROR, name));
        return;
      }
  
      Node secondArg = firstArg.getNext();
      name = secondArg.getOriginalName();
      if (name == null) {
        name = secondArg.getString();
      }
      JsMessage secondMessage = getTrackedMessage(t, name);
      if (secondMessage == null) {
        compiler.report(JSError.make(secondArg, FALLBACK_ARG_ERROR, name));
        return;
      }
  
      processMessageFallback(call, firstMessage, secondMessage);
    }
docs: Visit a call to goog.getMsgWithFallback.