Log more information to debug 'null' location for constructor.

pkg/analysis_server/lib/src/protocol_server.dart

Index: pkg/analysis_server/lib/src/protocol_server.dart
diff --git a/pkg/analysis_server/lib/src/protocol_server.dart b/pkg/analysis_server/lib/src/protocol_server.dart
index c1ebd1b7eca3bbd0b29514e61a4f25a143f0b1c7..9f12f18a1ac72765b32351d9980f4499d306ab4f 100644
--- a/pkg/analysis_server/lib/src/protocol_server.dart
+++ b/pkg/analysis_server/lib/src/protocol_server.dart
@@ -230,6 +230,19 @@ NavigationTarget newNavigationTarget_fromElement(engine.Element element, int
     fileToIndex(String file)) {
   ElementKind kind = newElementKind_fromEngine(element.kind);
   Location location = newLocation_fromElement(element);
+  // TODO(scheglov) debug null Location
+  if (location == null) {
+    String desc = 'location == null';
+    try {
+      desc += ' for: $element';
+      desc += ' of type: ${element.runtimeType}';
+      desc += ' element.location: ${element.location}';
+      desc += ' element.context: ${element.context}';
+      desc += ' element.source: ${element.source}';
+    } catch (e) {
+    }
+    throw new ArgumentError(desc);
+  }
   String file = location.file;
   int fileIndex = fileToIndex(file);
   return new NavigationTarget(