fix for-head duplication testing and replacement - fixes #162

test/fixtures/transformation/let-scoping/exec-block-scoped/exec.js

@@ -0,0 +1,6 @@
+let x = 1;
+{
+  let x = 2;
+  assert.equal(x, 2);
+}
+assert.equal(x, 1);

test/fixtures/transformation/let-scoping/exec-for-loop-head/exec.js

@@ -0,0 +1,5 @@
+assert.equal((function(){
+  let a = 1;
+  for (let a = 0; a < 8; a++) {}
+  return a;
+}()), 1);