add AssignmentPattern support to scope tracking - fixes #537

test/fixtures/transformation/es6-let-scoping/exec-destructuring-defaults/exec.js

@@ -0,0 +1,5 @@
+var fields = [{ name: "title" }, { name: "content" }];
+
+for (let { name, value = "Default value" } of fields) {
+  assert.equal(value, "Default value");
+}