What did Mr Lash president of the World Resources Institute in Washington mean when he said poor countries do not need to pass through the valley of the shadow of pollution?

Well since many countries have rapidly introduced industrialization it caused pollution which cost lots of money and when you clean it up it caused people to back into poverty.

He was commenting that the process of industrialization does not have to exactly follow that which was mapped out by the first industrial countries.

By that, he meant that there were many shortcuts possible in the process of industrialization, and that, with proper planning, many of the mistakes made by the first industrializing countries didn't have to be repeated. Most of those mistakes have lead to severe environmental pollution, and he was stating that a poor country could industrialize successfully without having to make these same pollution-causing mistakes.