Bug 1091113 - Don't merge obsolete entity fields. r=stas

bindings/l20n/buildtime.js

@@ -83,6 +83,8 @@ L10n.Context.prototype.getEntitySource = function(id) {
     throw new L10n.Error('Context not ready');
   }
 
+  var sourceEntity = this.getLocale('en-US').astById[id];
+
   var cur = 0;
   var loc;
   var locale;
@@ -94,7 +96,12 @@ L10n.Context.prototype.getEntitySource = function(id) {
     }
 
     if (locale.astById && id in locale.astById) {
-      return locale.astById[id];
+      var entity = locale.astById[id];
+      if (loc === 'en-US' || areEntityStructsEqual(sourceEntity, entity)) {
+        return entity;
+      } else {
+        return sourceEntity;
+      }
     }
 
     var e = new L10n.Error(id + ' not found in ' + loc, id, loc);
@@ -154,3 +161,20 @@ function flushBuildMessages(variant) {
     }
   }
 }
+
+function areEntityStructsEqual(entity1, entity2) {
+  var keys1 = Object.keys(entity1);
+  var keys2 = Object.keys(entity2);
+
+  if (keys1.length !== keys2.length) {
+    return false;
+  }
+
+  for (var i = 0; i < keys1.length; i++) {
+    if (keys2.indexOf(keys1[i]) === -1) {
+      return false;
+    }
+  }
+
+  return true;
+}