Fixed import errors.

src/dataflow.py

 from copy import copy
 
-from utils import Block
+from statement import Block
 
 
 class BasicBlock(Block):

tests/test_optimize.py

 import unittest
 
 from src.optimize import optimize_global
-from src.utils import Statement as S, Block as B
+from src.statement import Statement as S, Block as B
 
 
 class TestOptimize(unittest.TestCase):