Icon refinements for Dart elements (WEB-14083).
Dart documentation URL fixes.
NPE in DartDocumentationProvider and minor optimization
Dart documentation getter/setter URL fixes.
Merge branch 'structure_icons_14083' of https://github.com/pq/intellij-plugins
Merge branch 'dart_setter_doc_url_fix' of https://github.com/pq/intellij-plugins