allow Subscriber render out of context as a noop

need to talk to @mjackson about tradeoffs here, I’m undecided, but to
avoid breaking Router, we need this for now, then we can decide to keep
it this way or not

Author: ryanflorence

modules/Subscriber.js

@@ -16,19 +16,22 @@ class Subscriber extends React.Component {
 
   componentWillMount() {
     const { channel } = this.props
-    const subscribe = this.context.broadcasts[channel]
-
-    invariant(
-      typeof subscribe === 'function',
-      '<Subscriber channel="%s"> must be rendered in the context of a <Broadcast channel="%s">',
-      channel,
-      channel
-    )
-
-    this.unsubscribe = subscribe(value => {
-      // This function will be called once immediately.
-      this.setState({ value })
-    })
+
+    if (this.context.broadcasts) {
+      const subscribe = this.context.broadcasts[channel]
+
+      invariant(
+        typeof subscribe === 'function',
+        '<Subscriber channel="%s"> must be rendered in the context of a <Broadcast channel="%s">',
+        channel,
+        channel
+      )
+
+      this.unsubscribe = subscribe(value => {
+        // This function will be called once immediately.
+        this.setState({ value })
+      })
+    }
   }
 
   componentWillUnmount() {