test(@angular-devkit/core): fix memory host spec

This previously caused an `UnhandledPromiseRejection` which caused tests not to run. But Jasmine, still reported success.

```
yarn bazel test //packages/angular_devkit/core:core_test --test_summary=detailed --test_output=all

INFO: Analyzed target //packages/angular_devkit/core:core_test (1 packages loaded, 10 targets configured).
INFO: Found 1 test target...
Target //packages/angular_devkit/core:core_test up-to-date:
  dist/bin/packages/angular_devkit/core/core_test.sh
  dist/bin/packages/angular_devkit/core/core_test_loader.js
  dist/bin/packages/angular_devkit/core/core_test_require_patch.js
INFO: Elapsed time: 0.327s, Critical Path: 0.03s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
PASSED: //packages/angular_devkit/core:core_test (see /private/var/tmp/_bazel_alanagius/5168427e57f204ca069c602aa7ed1931/execroot/angular_cli/bazel-out/darwin-fastbuild/testlogs/packages/angular_devkit/core/core_test/test.log)
INFO: From Testing //packages/angular_devkit/core:core_test
==================== Test output for //packages/angular_devkit/core:core_test:
(node:12829) UnhandledPromiseRejectionWarning: Error: Cannot find module '/private/var/tmp/_bazel_alanagius/5168427e57f204ca069c602aa7ed1931/execroot/angular_cli/node_modules/@angular-devkit/core/src/index.js'. Please verify that the package.json has a valid "main" entry
    at tryPackage (internal/modules/cjs/loader.js:303:19)
    at Function.Module._findPath (internal/modules/cjs/loader.js:516:18)
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:867:27)
    at Function.Module._load (internal/modules/cjs/loader.js:725:27)
    at Module.require (internal/modules/cjs/loader.js:952:19)
    at require (internal/modules/cjs/helpers.js:88:18)
    at Object.<anonymous> (packages/angular_devkit/core/src/virtual-fs/host/memory_spec.ts:10:1)
    at Module._compile (internal/modules/cjs/loader.js:1063:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
    at Module.load (internal/modules/cjs/loader.js:928:32)
    at Function.Module._load (internal/modules/cjs/loader.js:769:14)
    at Module.require (internal/modules/cjs/loader.js:952:19)
    at require (internal/modules/cjs/helpers.js:88:18)
    at Loader.requireShim [as require_] (node_modules/jasmine/lib/loader.js:35:3)
    at node_modules/jasmine/lib/loader.js:28:12
    at new Promise (<anonymous>)
    at Loader.load (node_modules/jasmine/lib/loader.js:27:12)
    at Jasmine.loadSpecs (node_modules/jasmine/lib/jasmine.js:91:23)
    at Jasmine.execute (node_modules/jasmine/lib/jasmine.js:267:3)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:12829) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)
(node:12829) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
================================================================================
Test cases: finished with 1 passing and 0 failing out of 1 test cases

Executed 0 out of 1 test: 1 test passes.
```

packages/angular_devkit/core/src/virtual-fs/host/memory_spec.ts

@@ -7,7 +7,7 @@
  */
 
 /* eslint-disable @typescript-eslint/no-non-null-assertion */
-import { fragment, normalize } from '@angular-devkit/core';
+import { fragment, normalize } from '../path';
 import { stringToFileBuffer } from './buffer';
 import { SimpleMemoryHost } from './memory';
 import { SyncDelegateHost } from './sync';