Update loading pre-bundled message

Summary:
Updated the message from

> Loading from pre-bundled file

to

> Connect to Metro to develop JavaScript

I also added a new RCT_PACKAGER_NAME so other packagers can override "Metro"

Reviewed By: yungsters, cpojer

Differential Revision: D16427501

fbshipit-source-id: 1b7f9e261f7521ba930c6248087fe6f3c3659cb7

React/Base/RCTDefines.h

@@ -89,6 +89,13 @@
 #endif
 #endif
 
+/**
+ * Add the default packager name
+ */
+#ifndef RCT_PACKAGER_NAME
+#define RCT_PACKAGER_NAME @"Metro"
+#endif
+
 /**
  * By default, only raise an NSAssertion in debug mode
  * (custom assert functions will still be called).

React/DevSupport/RCTDevLoadingView.m

@@ -130,22 +130,22 @@ - (void)showWithURL:(NSURL *)URL
 {
   UIColor *color;
   UIColor *backgroundColor;
-  NSString *source;
+  NSString *message;
   if (URL.fileURL) {
     // If dev mode is not enabled, we don't want to show this kind of notification
 #if !RCT_DEV
     return;
 #endif
-    color = [UIColor grayColor];
+    color = [UIColor whiteColor];
     backgroundColor = [UIColor blackColor];
-    source = @"pre-bundled file";
+      message = [NSString stringWithFormat:@"Connect to %@ to develop JavaScript.", RCT_PACKAGER_NAME];
   } else {
     color = [UIColor whiteColor];
     backgroundColor = [UIColor colorWithHue:1./3 saturation:1 brightness:.35 alpha:1];
-    source = [NSString stringWithFormat:@"%@:%@", URL.host, URL.port];
+    message = [NSString stringWithFormat:@"Loading from %@:%@...", URL.host, URL.port];
   }
 
-  [self showMessage:[NSString stringWithFormat:@"Loading from %@...", source]
+  [self showMessage:message
               color:color
     backgroundColor:backgroundColor];
 }