fix(Symbol): $$observable is not defined

Author: bouzuya

src/symbol/observable.ts

@@ -5,7 +5,9 @@ const Symbol: any = root.Symbol;
 export let $$observable: any;
 
 if (typeof Symbol === 'function') {
-  if (!Symbol.observable) {
+  if (Symbol.observable) {
+    $$observable = Symbol.observable;
+  } else {
     if (typeof Symbol.for === 'function') {
       $$observable = Symbol.for('observable');
     } else {
@@ -15,4 +17,4 @@ if (typeof Symbol === 'function') {
   }
 } else {
   $$observable = '@@observable';
-}
+}