[test] Add failing test of incorrect typing of captured ref

The typer doesn't understand that it needs to ignore captured references ("read" 
refs are ok, but not "mutated" refs) to be conservative.

compiler/forget/src/__tests__/fixtures/hir/_bug.capturing-reference-changes-type.expect.md

@@ -0,0 +1,31 @@
+
+## Input
+
+```javascript
+function component(a) {
+  let x = { a };
+  let y = 1;
+  (function () {
+    y = x;
+  })();
+  mutate(y);
+  return y;
+}
+
+```
+
+## Code
+
+```javascript
+function component(a) {
+  const x = { a: a };
+
+  (function () {
+    y = x;
+  })();
+  mutate(1);
+  return 1;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/_bug.capturing-reference-changes-type.js

@@ -0,0 +1,9 @@
+function component(a) {
+  let x = { a };
+  let y = 1;
+  (function () {
+    y = x;
+  })();
+  mutate(y);
+  return y;
+}