address comments

src/cdk/testing/harness-environment.ts

@@ -90,7 +90,7 @@ export abstract class HarnessEnvironment<E> implements HarnessLoader, LocatorFac
   // Implemented as part of the `LocatorFactory` interface.
   async harnessLoaderForOptional(selector: string): Promise<HarnessLoader | null> {
     const elements = await this.getAllRawElements(selector);
-    return elements[0] && this.createEnvironment(elements[0]);
+    return elements[0] ? this.createEnvironment(elements[0]) : null;
   }
 
   // Implemented as part of the `LocatorFactory` interface.

src/material/tabs/testing/shared.spec.ts

@@ -69,7 +69,7 @@ export function runHarnessTests(
   it('should throw error when attempting to select invalid tab', async () => {
     const tabGroup = await loader.getHarness(tabGroupHarness);
     await expectAsyncError(() => tabGroup.selectTab({label: 'Fake'}),
-        /Error: Cannot find mat-tab matching {"label":"Fake"}/);
+        /Error: Cannot find mat-tab matching filter {"label":"Fake"}/);
   });
 
   it('should be able to get label of tabs', async () => {

src/material/tabs/testing/tab-group-harness.ts

@@ -55,7 +55,7 @@ export class MatTabGroupHarness extends ComponentHarness {
   async selectTab(filter: TabHarnessFilters = {}): Promise<void> {
     const tabs = await this.getTabs(filter);
     if (!tabs.length) {
-      throw Error(`Cannot find mat-tab matching ${JSON.stringify(filter)}`);
+      throw Error(`Cannot find mat-tab matching filter ${JSON.stringify(filter)}`);
     }
     await tabs[0].select();
   }

src/material/tabs/testing/tab-harness.ts

@@ -60,6 +60,10 @@ export class MatTabHarness extends ComponentHarness {
     await (await this.host()).click();
   }
 
+  /**
+   * Gets a `HarnessLoader` that can be used to load harnesses for components within the tab's
+   * content area.
+   */
   async getHarnessLoaderForContent(): Promise<HarnessLoader> {
     const contentId = await this._getContentId();
     return this.documentRootLocatorFactory().harnessLoaderFor(`#${contentId}`);