fix(es5): throw if a modified child draft is returned

To be more consistent with the native Proxy implementation.

__tests__/base.js

@@ -794,14 +794,25 @@ function runBaseTest(name, useProxies, autoFreeze, useListener) {
                 expect(res).toEqual({x: 4})
             })
 
-            it("can return a child draft", () => {
+            it("can return an unmodified child draft", () => {
                 const base = {a: {}}
                 const res = produce(base, d => {
                     return d.a
                 })
                 expect(res).toBe(base.a)
             })
 
+            // TODO: Avoid throwing if only the child draft was modified.
+            it("cannot return a modified child draft", () => {
+                const base = {a: {}}
+                expect(() => {
+                    produce(base, d => {
+                        d.a.b = 1
+                        return d.a
+                    })
+                }).toThrow(/^An immer producer returned/)
+            })
+
             it("can return a frozen object", () => {
                 const res = deepFreeze([{x: 3}])
                 expect(produce({}, () => res)).toBe(res)

src/es5.js

@@ -26,6 +26,10 @@ export function willFinalize(result, baseDraft, needPatches) {
         // This is faster when we don't care about which attributes changed.
         markChangesSweep(scope)
     }
+    // When a child draft is returned, look for changes.
+    else if (isDraft(result) && result[DRAFT_STATE].scope === scope) {
+        markChangesSweep(scope.drafts)
+    }
 }
 
 export function createDraft(base, parent) {