forbid function declarations that reference block scoped variables

test/fixtures/syntax/let-scoping/no-nested-function-declarations/actual.js

@@ -0,0 +1,7 @@
+var numbers = [1, 2, 3];
+
+for (let i in numbers) {
+  function foo() {
+    return i;
+  }
+}

test/fixtures/syntax/let-scoping/no-nested-function-declarations/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "`FunctionDeclaration`s that use `let` and `constant` references aren't allowed outside of the root scope"
+}