only import fake-indexeddb in tests

as it is a devDependency and can end up in the legacy bundle
under circumstances

src/matrix/storage/idb/QueryTarget.ts

@@ -269,13 +269,18 @@ export class QueryTarget<T> {
     }
 }
 
-import {createMockDatabase, MockIDBImpl} from "../../../mocks/Storage";
+import {createMockDatabase, createMockIDBFactory, getMockIDBKeyRange} from "../../../mocks/Storage";
 import {txnAsPromise} from "./utils";
 import {QueryTargetWrapper, Store} from "./Store";
 
-export function tests() {
+export async function tests() {
 
-    class MockTransaction extends MockIDBImpl {
+    class MockTransaction {
+        constructor(public readonly idbFactory: IDBFactory, readonly idbKeyRangeType: typeof IDBKeyRange) {}
+
+        get IDBKeyRange(): typeof IDBKeyRange {
+            return this.idbKeyRangeType;
+        }
         get databaseName(): string { return "mockdb"; }
         addWriteError(error: StorageError, refItem: ILogItem | undefined, operationName: string, keys: IDBKey[] | undefined) {}
     }
@@ -285,10 +290,12 @@ export function tests() {
     }
 
     async function createTestStore(): Promise<Store<TestEntry>> {
-        const mockImpl = new MockTransaction();
+        const idbFactory = await createMockIDBFactory();
+        const idbKeyRangeType = await getMockIDBKeyRange();
+        const mockImpl = new MockTransaction(idbFactory, idbKeyRangeType);
         const db = await createMockDatabase("findExistingKeys", (db: IDBDatabase) => {
             db.createObjectStore("test", {keyPath: "key"});
-        }, mockImpl);
+        }, idbFactory);
         const txn = db.transaction(["test"], "readwrite");
         return new Store<TestEntry>(txn.objectStore("test"), mockImpl);
     }

src/mocks/Storage.ts

@@ -14,34 +14,35 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-// @ts-ignore
-import FDBFactory from "fake-indexeddb/lib/FDBFactory.js";
-// @ts-ignore
-import FDBKeyRange from "fake-indexeddb/lib/FDBKeyRange.js";
 import {StorageFactory} from "../matrix/storage/idb/StorageFactory";
 import {IDOMStorage} from "../matrix/storage/idb/types";
 import {Storage} from "../matrix/storage/idb/Storage";
 import {Instance as nullLogger} from "../logging/NullLogger";
 import {openDatabase, CreateObjectStore} from "../matrix/storage/idb/utils";
 
-export function createMockStorage(): Promise<Storage> {
-    return new StorageFactory(null as any, new FDBFactory(), FDBKeyRange, new MockLocalStorage()).create("1", nullLogger.item);
+export async function createMockStorage(): Promise<Storage> {
+    const idbFactory = await createMockIDBFactory();
+    const FDBKeyRange = await getMockIDBKeyRange();
+    return new StorageFactory(null as any, idbFactory, FDBKeyRange, new MockLocalStorage()).create("1", nullLogger.item);
 }
 
-export function createMockDatabase(name: string, createObjectStore: CreateObjectStore, impl: MockIDBImpl): Promise<IDBDatabase> {
-    return openDatabase(name, createObjectStore, 1, impl.idbFactory);
+// don't import fake-indexeddb until it's safe to assume we're actually in a unit test,
+// as this is a devDependency
+export async function createMockIDBFactory(): Promise<IDBFactory> {
+    // @ts-ignore
+    const FDBFactory = (await import("fake-indexeddb/lib/FDBFactory.js")).default;
+    return new FDBFactory();
 }
 
-export class MockIDBImpl {
-    idbFactory: FDBFactory;
+// don't import fake-indexeddb until it's safe to assume we're actually in a unit test,
+// as this is a devDependency
+export async function getMockIDBKeyRange(): Promise<typeof IDBKeyRange> {
+    // @ts-ignore
+    return (await import("fake-indexeddb/lib/FDBKeyRange.js")).default;
+}
 
-    constructor() {
-        this.idbFactory = new FDBFactory();
-    }
-
-    get IDBKeyRange(): typeof IDBKeyRange {
-        return FDBKeyRange;
-    }
+export function createMockDatabase(name: string, createObjectStore: CreateObjectStore, idbFactory: IDBFactory): Promise<IDBDatabase> {
+    return openDatabase(name, createObjectStore, 1, idbFactory);
 }
 
 class MockLocalStorage implements IDOMStorage {