Revert "chore: delete import that was missed in #18274 (#18334)"

This reverts commit 7794fe8.

Author: mmalerba

src/cdk/testing/testbed/proxy-zone-types.d.ts

@@ -16,6 +16,8 @@
  * be pulled in from the original type definitions.
  */
 
+import {HasTaskState, Zone, ZoneDelegate} from './zone-types';
+
 export interface ProxyZoneStatic {
   assertPresent: () => ProxyZone;
   get(): ProxyZone;

src/cdk/tsconfig-tests.json

@@ -20,8 +20,7 @@
   },
   "include": [
     "**/index.ts",
-    "**/*.spec.ts",
-    "**/*.d.ts"
+    "**/*.spec.ts"
   ],
   "exclude": [
     "testing/protractor/**.ts",