Async initialization for DateFormat

pkg/intl/data/dates/symbols/en_GB.json

Index: pkg/intl/data/dates/symbols/en_GB.json
===================================================================
--- pkg/intl/data/dates/symbols/en_GB.json	(revision 0)
+++ pkg/intl/data/dates/symbols/en_GB.json	(revision 0)
@@ -0,0 +1 @@
+{"DATEFORMATS":["EEEE, d MMMM y","d MMMM y","d MMM y","dd/MM/yyyy"],"NARROWWEEKDAYS":["S","M","T","W","T","F","S"],"STANDALONESHORTMONTHS":["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"],"ERANAMES":["Before Christ","Anno Domini"],"FIRSTWEEKCUTOFFDAY":3,"NARROWMONTHS":["J","F","M","A","M","J","J","A","S","O","N","D"],"STANDALONESHORTWEEKDAYS":["Sun","Mon","Tue","Wed","Thu","Fri","Sat"],"FIRSTDAYOFWEEK":0,"STANDALONEWEEKDAYS":["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"],"WEEKDAYS":["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"],"QUARTERS":["1st quarter","2nd quarter","3rd quarter","4th quarter"],"ERAS":["BC","AD"],"MONTHS":["January","February","March","April","May","June","July","August","September","October","November","December"],"TIMEFORMATS":["HH:mm:ss zzzz","HH:mm:ss z","HH:mm:ss","HH:mm"],"SHORTWEEKDAYS":["Sun","Mon","Tue","Wed","Thu","Fri","Sat"],"SHORTQUARTERS":["Q1","Q2","Q3","Q4"],"WEEKENDRANGE":[5,6],"STANDALONENARROWWEEKDAYS":["S","M","T","W","T","F","S"],"AVAILABLEFORMATS":null,"NAME":"en_GB","AMPMS":["AM","PM"],"STANDALONEMONTHS":["January","February","March","April","May","June","July","August","September","October","November","December"],"SHORTMONTHS":["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"],"STANDALONENARROWMONTHS":["J","F","M","A","M","J","J","A","S","O","N","D"]}