build: prepare for TypeScript 3.9 (#36989)

- Fix several compilation errors
- Update @microsoft/api-extractor to be compatible with TypeScript 3.9

PR Close #36989

packages/forms/test/form_control_spec.ts

@@ -36,7 +36,7 @@ function asyncValidator(expected: string, timeouts = {}): AsyncValidatorFn {
 }
 
 function asyncValidatorReturningObservable(c: AbstractControl) {
-  const e = new EventEmitter();
+  const e = new EventEmitter<Record<string, boolean>>();
   Promise.resolve(null).then(() => {
     e.emit({'async': true});
   });