fix(material-experimental/mdc-slider): correct description of slider … (#23431)

* fix(material-experimental/mdc-slider): correct description of slider harness thumb getters

* fix(material-experimental/mdc-slider): throw helpful error when wrong slider thumb getter is used

* added a unit test to make sure this error works properly

(cherry picked from commit c4cf3c9)

Author: wagnermaciel

src/material-experimental/mdc-slider/testing/slider-harness.spec.ts

@@ -68,6 +68,14 @@ describe('MDC-based MatSliderHarness', () => {
     expect(await sliders[1].getEndThumb()).toBeTruthy();
   });
 
+  it('should throw when trying to get the start thumb from a single point slider', async () => {
+    const slider = await loader.getHarness(MatSliderHarness.with({isRange: false}));
+    await expectAsync(slider.getStartThumb()).toBeRejectedWithError(
+        '`getStartThumb` is only applicable for range sliders. '
+        + 'Did you mean to use `getEndThumb`?',
+      );
+  });
+
   it('should get the step of a slider', async () => {
     const sliders = await loader.getAllHarnesses(MatSliderHarness);
     expect(await parallel(() => {

src/material-experimental/mdc-slider/testing/slider-harness.ts

@@ -28,12 +28,18 @@ export class MatSliderHarness extends ComponentHarness {
       });
   }
 
-  /** Gets the start/primary thumb of the slider. */
+  /** Gets the start thumb of the slider (only applicable for range sliders). */
   async getStartThumb(): Promise<MatSliderThumbHarness> {
+    if (!await this.isRange()) {
+      throw Error(
+        '`getStartThumb` is only applicable for range sliders. '
+        + 'Did you mean to use `getEndThumb`?'
+      );
+    }
     return this.locatorFor(MatSliderThumbHarness.with({position: ThumbPosition.START}))();
   }
 
-  /** Gets the end thumb of the slider. Will throw an error for a non-range slider. */
+  /** Gets the thumb (for single point sliders), or the end thumb (for range sliders). */
   async getEndThumb(): Promise<MatSliderThumbHarness> {
     return this.locatorFor(MatSliderThumbHarness.with({position: ThumbPosition.END}))();
   }