types(reactivity): remove this in get accessor, fix #800 (#806)

packages/reactivity/src/baseHandlers.ts

@@ -23,7 +23,7 @@ const arrayInstrumentations: Record<string, Function> = {}
     for (let i = 0, l = (this as any).length; i < l; i++) {
       track(arr, TrackOpTypes.GET, i + '')
     }
-    // we run the method using the orignal args first (which may be reactive)
+    // we run the method using the original args first (which may be reactive)
     const res = arr[key](...args)
     if (res === -1 || res === false) {
       // if that didn't work, run it again using raw values.

packages/reactivity/src/collectionHandlers.ts

@@ -200,8 +200,8 @@ const readonlyInstrumentations: Record<string, Function> = {
   get(this: MapTypes, key: unknown) {
     return get(this, key, toReadonly)
   },
-  get size(this: IterableCollections) {
-    return size(this)
+  get size() {
+    return size((this as unknown) as IterableCollections)
   },
   has,
   add: createReadonlyMethod(add, TriggerOpTypes.ADD),