(refactor/test): only ignore parts of AsyncLocalStorage

- instead of the whole file, just clear and removeItem, as those two
  aren't currently used internally
  - have to use this funky style comment-in-the-middle-of-function-
    declaration for now as it won't ignore the func otherwise,
    possibly because of how it gets transpiled down to ES5

- invalid usage of localStorage functions also isn't done internally,
  so ignore the catch / Promise.reject block as well

src/asyncLocalStorage.ts

@@ -1,3 +1,5 @@
+// using istanbul ignore on portions of code that are not currently used internally
+
 interface IAsyncLocalStorage {
   clear(): Promise<void>
   getItem(key: string): Promise<string>
@@ -7,13 +9,13 @@ interface IAsyncLocalStorage {
 
 export const AsyncLocalStorage: IAsyncLocalStorage = {
   // must use wrapper functions when passing localStorage functions (https://github.com/agilgur5/mst-persist/issues/18)
-  clear () {
+  clear /* istanbul ignore next */ () {
     return callWithPromise(() => window.localStorage.clear())
   },
   getItem (key) {
     return callWithPromise(() => window.localStorage.getItem(key))
   },
-  removeItem (key) {
+  removeItem /* istanbul ignore next */ (key) {
     return callWithPromise(() => window.localStorage.removeItem(key))
   },
   setItem (key, value) {
@@ -25,6 +27,7 @@ function callWithPromise (func: Function, ...args: any[]): Promise<any> {
   try {
     return Promise.resolve(func(...args))
   } catch (err) {
+    /* istanbul ignore next */
     return Promise.reject(err)
   }
 }