feat(cdk-experimental/testing): Add a public stabilize method to TestElement (#16847)

* feat(cdk-experimental/testing): Add a public stabilize method to
TestElement

* rename to `forceStabilize`

Author: mmalerba

src/cdk-experimental/testing/protractor/protractor-element.ts

@@ -141,4 +141,6 @@ export class ProtractorElement implements TestElement {
   async getProperty(name: string): Promise<any> {
     return browser.executeScript(`return arguments[0][arguments[1]]`, this.element, name);
   }
+
+  async forceStabilize(): Promise<void> {}
 }

src/cdk-experimental/testing/test-element.ts

@@ -90,10 +90,7 @@ export interface TestElement {
   /** Gets the text from the element. */
   text(): Promise<string>;
 
-  /**
-   * Gets the value for the given attribute from the element. If the attribute does not exist,
-   * falls back to reading the property.
-   */
+  /** Gets the value for the given attribute from the element. */
   getAttribute(name: string): Promise<string | null>;
 
   /** Checks whether the element has the given class. */
@@ -104,4 +101,11 @@ export interface TestElement {
 
   /** Gets the value of a property of an element. */
   getProperty(name: string): Promise<any>;
+
+  /**
+   * Flushes change detection and async tasks.
+   * In most cases it should not be necessary to call this. However, there may be some edge cases
+   * where it is needed to fully flush animation events.
+   */
+  forceStabilize(): Promise<void>;
 }

src/cdk-experimental/testing/testbed/unit-test-element.ts

@@ -137,4 +137,8 @@ export class UnitTestElement implements TestElement {
     await this._stabilize();
     return (this.element as any)[name];
   }
+
+  async forceStabilize(): Promise<void> {
+    return this._stabilize();
+  }
 }