[rxjs] Establish interop of rxjs-6 and kbn-observable (elastic#21722)

* [rxjs] Establish interop of rxjs-6 and kbn-observable
* Remove symbol-observable

package.json

@@ -175,7 +175,6 @@
     "script-loader": "0.7.2",
     "semver": "^5.5.0",
     "style-loader": "0.19.0",
-    "symbol-observable": "^1.2.0",
     "tar": "2.2.0",
     "tinygradient": "0.3.0",
     "tinymath": "0.2.1",

src/core/lib/kbn_internal_native_observable/index.js

@@ -1,4 +1,4 @@
-import symbolObservable from 'symbol-observable';
+import { observable as SymbolObservable } from 'rxjs/internal/symbol/observable';
 
 // This is a fork of the example implementation of the TC39 Observable spec,
 // see https://github.com/tc39/proposal-observable.
@@ -256,7 +256,7 @@ export class Observable {
     return new Subscription(observer, this._subscriber);
   }
 
-  [symbolObservable]() { return this }
+  [SymbolObservable]() { return this }
 
   // == Derived ==
 
@@ -267,7 +267,7 @@ export class Observable {
     if (x == null)
       throw new TypeError(x + " is not an object");
 
-    let method = getMethod(x, symbolObservable);
+    let method = getMethod(x, SymbolObservable);
 
     if (method) {
 

src/core/lib/kbn_observable/__tests__/observable_rxjs.test.ts

@@ -0,0 +1,37 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import { from, of } from 'rxjs';
+import { isObservable } from '../lib/is_observable';
+import { Observable } from '../observable';
+
+// Test that rxjs observable and kbn-observable are interoperable.
+describe('interoperability', () => {
+  it('understood by rxjs of kbn-observables', () => {
+    const obs = Observable.of([1, 2, 3]);
+    expect(() => {
+      from(obs);
+    }).not.toThrowError(TypeError);
+  });
+
+  it('understood by kbn-observable of rxjs observables', () => {
+    const rxobs = of([1, 2, 3]);
+    expect(isObservable(rxobs)).toBeTruthy();
+  });
+});

src/core/lib/kbn_observable/lib/is_observable.ts

@@ -17,8 +17,9 @@
  * under the License.
  */
 
+import { observable as SymbolObservable } from 'rxjs/internal/symbol/observable';
 import { Observable } from '../observable';
 
 export function isObservable<T>(x: any): x is Observable<T> {
-  return x !== null && typeof x === 'object' && x[Symbol.observable] !== undefined;
+  return x !== null && typeof x === 'object' && x[SymbolObservable] !== undefined;
 }