code: private void addBlockToAddedBlocks(long blockId, String tierAlias) {
      if (mAddedBlocks.containsKey(tierAlias)) {
        mAddedBlocks.get(tierAlias).add(blockId);
      } else {
        mAddedBlocks.put(tierAlias, Lists.newArrayList(blockId));
      }
    }
docs: Adds a block to the list of added blocks in this heartbeat period.
  
  @param blockId the id of the block to add
  @param tierAlias alias of the storage tier containing the block