revamp let scoping transformer - closes #510

test/fixtures/transformation/es6-let-scoping/exec-block-scoped-2/exec.js

@@ -0,0 +1,12 @@
+assert.equal(() => {
+  let sum = 0;
+  let a = 0;
+  {
+    let a = 10;
+    for (let i = 0; i < a; i++) {
+      let a = 1;
+      sum += (() => a)();
+    }
+  }
+  return sum;
+}(), 10);