ImportFlatShadedMesh.cs

using UnityEditor;
using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class ImportFlatShadedMesh : AssetPostprocessor {
	void OnPostprocessModel(GameObject g){
		Mesh m = g.GetComponentInChildren<MeshFilter>().sharedMesh;

		Vector3[] oldVertices = m.vertices;
		Vector3[] newVertices = new Vector3[m.triangles.Length];
		Vector2[] newUvs1 = new Vector2[newVertices.Length];
		Vector2[] newUvs2 = new Vector2[newVertices.Length];
		int uv1l = m.uv.Length;
		int uv2l = m.uv2.Length;
		
		// Loop over submeshes
		int offset = 0;
		int[][] submeshes = new int[m.subMeshCount][];
		for (int s=0; s<m.subMeshCount; ++s){
			submeshes[s] = m.GetIndices(s);
			for(int i = 0; i < submeshes[s].Length; i++) {
				newVertices[i+offset] = oldVertices[submeshes[s][i]];
				if (uv1l<=submeshes[s][i]){
					newUvs1[i+offset] = new Vector2(0,0);
				} else {
					newUvs1[i+offset] = m.uv[submeshes[s][i]];
				}
				if (uv2l<=submeshes[s][i]){
					newUvs2[i+offset] = new Vector2(0,0);
				} else {
					newUvs2[i+offset] = m.uv2[submeshes[s][i]];
				}
				submeshes[s][i] = i+offset;
			}
			offset += submeshes[s].Length;
		}
		
		// re-assign to mesh
		m.vertices = newVertices;
		for (int i=0; i<submeshes.Length; ++i){
			m.SetTriangles(submeshes[i],i);
		}
		m.uv = newUvs1;
		m.uv2 = newUvs2;
		m.RecalculateNormals();
	}
}