Allow analyzer 0.26.0-alpha.1.
Allow analyzer 0.26.0-alpha.1.

R=jmesserly@google.com

Review URL: https://chromiumcodereview.appspot.com//1260143002 .

alechenninger/dart_style,christofferqa/dart_style,mark-henry/dart_style,alechenninger/dart_style,christofferqa/dart_style,dart-lang/dart_style,mark-henry/dart_style