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

pkg/analysis_server/lib/src/protocol_server.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library protocol.server;
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_server/src/services/search/search_engine.dart' as
     engine;
 import 'package:analyzer/src/generated/ast.dart' as engine;
@@ skipping 212 matching lines @@
   engine.AnalysisContext context = unitElement.context;
   engine.Source source = unitElement.source;
   return _locationForArgs(context, source, range);
 }
 
 
 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(
       kind,
       fileIndex,
       location.offset,
       location.length,
       location.startLine,
       location.startColumn);
 }
@@ skipping 179 matching lines @@
       startColumn = offsetLocation.columnNumber;
     }
   }
   return new Location(
       source.fullName,
       range.offset,
       range.length,
       startLine,
       startColumn);
 }