Add SearchEngine.searchElementDeclarations().

R=brianwilkerson@google.com
BUG=

Review URL: https://codereview.chromium.org//476383002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@39315 260f80e4-7a28-3924-810f-c04153c831b5
remove unused completion proposal classes and related methods

BUG=
R=messick@google.com, scheglov@google.com

Review URL: https://codereview.chromium.org//478763002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@39316 260f80e4-7a28-3924-810f-c04153c831b5