title: no exported member 'Observable'
body: <p>I am trying to use agm in my angular project 
  So I installed npm install @agm/core --save
  but when I use<br>
  AgmCoreModule.forRoot({
        apiKey: 'my api'
      }),
  It makes an error that I can not understand :</p>
  
  <pre><code>ERROR in node_modules/@agm/core/services/google-maps-api-wrapper.d.ts(2,10): error TS2305: Module '"C:/Users/Shadi/Documents/development/iot1/node_modules/rxjs/Observable"' has no exported member 'Observable'.
  node_modules/@agm/core/services/managers/circle-manager.d.ts(2,10): error TS2305: Module '"C:/Users/Shadi/Documents/development/iot1/node_modules/rxjs/Observable"' has no exported member
   'Observable'.
  node_modules/@agm/core/services/managers/data-layer-manager.d.ts(2,10): error TS2305: Module '"C:/Users/Shadi/Documents/development/iot1/node_modules/rxjs/Observable"' has no exported me
  mber 'Observable'.
  node_modules/@agm/core/services/managers/info-window-manager.d.ts(1,10): error TS2305: Module '"C:/Users/Shadi/Documents/development/iot1/node_modules/rxjs/Observable"' has no exported m
  ember 'Observable'.
  node_modules/@agm/core/services/managers/kml-layer-manager.d.ts(2,10): error TS2305: Module '"C:/Users/Shadi/Documents/development/iot1/node_modules/rxjs/Observable"' has no exported mem
  ber 'Observable'.
  node_modules/@agm/core/services/managers/marker-manager.d.ts(2,10): error TS2305: Module '"C:/Users/Shadi/Documents/development/iot1/node_modules/rxjs/Observable"' has no exported member
   'Observable'.
  node_modules/@agm/core/services/managers/polygon-manager.d.ts(2,10): error TS2305: Module '"C:/Users/Shadi/Documents/development/iot1/node_modules/rxjs/Observable"' has no exported membe
  r 'Observable'.
  node_modules/@agm/core/services/managers/polyline-manager.d.ts(2,10): error TS2305: Module '"C:/Users/Shadi/Documents/development/iot1/node_modules/rxjs/Observable"' has no exported memb
  er 'Observable'.
  node_modules/rxjs/Observable.d.ts(1,15): error TS2307: Cannot find module 'rxjs-compat/Observable'.
  </code></pre>
  
  <p>this is appModule : </p>
  
  <pre><code>import { BrowserModule } from '@angular/platform-browser';
  import { NgModule } from '@angular/core';
  
  import { AppComponent } from './app.component';
  import { FwModule } from '../fw/fw.module';
  import { DashboardComponent } from './dashboard/dashboard.component';
  import { SettingsComponent } from './settings/settings.component';
  import {FormsModule} from '@angular/forms';
  import {RouterModule} from '@angular/router';
  import {appRoutes} from './app.routing';
  import { TagsComponent } from './tags/tags.component';
  import {DataService} from './shared/data.service';
  import {TagsResolverService} from './shared/tags-resolver.service';
  import {HttpClientModule} from '@angular/common/http';
  import {AgmCoreModule} from '@agm/core';
  
  
  
  @NgModule({
    declarations: [
      AppComponent,
      DashboardComponent,
      SettingsComponent,
      TagsComponent
    ],
    imports: [
      BrowserModule,
      FwModule,
      FormsModule,
      RouterModule.forRoot(appRoutes),
      HttpClientModule,
      AgmCoreModule.forRoot({
        apiKey: 'AIzaSyAt1d6t28Sbvaa4exq2dHKOffqUCEGXoqQ'
      }),
    ],
    providers: [
      DataService,
      TagsResolverService
    ],
    bootstrap: [AppComponent]
  })
  export class AppModule { }
  </code></pre>
  
  <p>I deleted node_module and installed again but the same error appear</p>