perf(reactivity): add existing index or non-integer prop on Array should not trigger length dependency (#1969)

packages/reactivity/__tests__/reactiveArray.spec.ts

@@ -99,6 +99,33 @@ describe('reactivity/reactive/Array', () => {
     expect(fn).toHaveBeenCalledTimes(1)
   })
 
+  test('add existing index on Array should not trigger length dependency', () => {
+    const array = new Array(3)
+    const observed = reactive(array)
+    const fn = jest.fn()
+    effect(() => {
+      fn(observed.length)
+    })
+    expect(fn).toHaveBeenCalledTimes(1)
+    observed[1] = 1
+    expect(fn).toHaveBeenCalledTimes(1)
+  })
+
+  test('add non-integer prop on Array should not trigger length dependency', () => {
+    const array = new Array(3)
+    const observed = reactive(array)
+    const fn = jest.fn()
+    effect(() => {
+      fn(observed.length)
+    })
+    expect(fn).toHaveBeenCalledTimes(1)
+    // @ts-ignore
+    observed.x = 'x'
+    expect(fn).toHaveBeenCalledTimes(1)
+    observed[-1] = 'x'
+    expect(fn).toHaveBeenCalledTimes(1)
+  })
+
   describe('Array methods w/ refs', () => {
     let original: any[]
     beforeEach(() => {