Move Bidi functions into methods to make the docs look better

pkg/intl/lib/src/intl_helpers.dart

Index: pkg/intl/lib/src/intl_helpers.dart
===================================================================
--- pkg/intl/lib/src/intl_helpers.dart	(revision 13251)
+++ pkg/intl/lib/src/intl_helpers.dart	(working copy)
@@ -40,4 +40,23 @@
  */
 abstract class LocaleDataReader {
   abstract Future read(String locale);
+}
+
+/**
+ * The internal mechanism for looking up messages. We expect this to be set
+ * by the implementing package so that we're not dependent on its
+ * implementation.
+ */
+var messageLookup = const
+    UninitializedLocaleData('initializeMessages(<locale>)');
+
+/**
+ * Initialize the message lookup mechanism. This is for internal use only.
+ * User applications should import `message_lookup_local.dart` and call
+ * `initializeMessages`
+ */
+void initializeInternalMessageLookup(Function lookupFunction) {
+  if (messageLookup is UninitializedLocaleData) {
+    messageLookup = lookupFunction();
+  }
 }