Fields in closures.

test/field/nested_class.wren

@@ -0,0 +1,18 @@
+class Outer {
+  method {
+    _field = "outer"
+    IO.write(_field) // expect: outer
+
+    class Inner {
+      method {
+        _field = "inner"
+        IO.write(_field) // expect: inner
+      }
+    }
+
+    (new Inner).method
+    IO.write(_field) // expect: outer
+  }
+}
+
+(new Outer).method