angular – 错误：’Observable’类型中不存在属性’pipe’

https://material.angular.io/components/autocomplete/overview

HTML：

<form class="example-form">
  <mat-form-field class="example-full-width">
    <input type="text" placeholder="Pick one" aria-label="Number" matInput [formControl]="myControl" [matAutocomplete]="auto">
    <mat-autocomplete #auto="matAutocomplete">
      <mat-option *ngFor="let option of filteredOptions | async" [value]="option">
        {{ option }}
      </mat-option>
    </mat-autocomplete>
  </mat-form-field>
</form>

TS：

import {Component} from '@angular/core';
import {FormControl} from '@angular/forms';
import {Observable} from 'rxjs/Observable';
import {startWith} from 'rxjs/operators/startWith';
import {map} from 'rxjs/operators/map';

/**
 * @title Filter autocomplete
 */
@Component({
  selector: 'autocomplete-filter-example',templateUrl: 'autocomplete-filter-example.html',styleUrls: ['autocomplete-filter-example.css']
})
export class AutocompleteFilterExample {

  myControl: FormControl = new FormControl();

  options = [
    'One','Two','Three'
  ];

  filteredOptions: Observable<string[]>;

  ngOnInit() {
    this.filteredOptions = this.myControl.valueChanges
      .pipe(
        startWith(''),map(val => this.filter(val))
      );
  }

  filter(val: string): string[] {
    return this.options.filter(option =>
      option.toLowerCase().indexOf(val.toLowerCase()) === 0);
  }

}

CSS：

.example-form {
  min-width: 150px;
  max-width: 500px;
  width: 100%;
}

.example-full-width {
  width: 100%;
}

但我得到这个错误：

Failed to compile: Property ‘pipe’ does not exist on type ‘Observable’.

知道为什么吗？