Fix analyzer_experimental hints: remove unused imports.

pkg/analyzer_experimental/lib/src/generated/source_io.dart

 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 library engine.source.io;
 import 'source.dart';
-import 'dart:io';
 import 'java_core.dart';
 import 'java_io.dart';
-import 'sdk.dart' show DartSdk;
 import 'engine.dart' show AnalysisContext, AnalysisEngine;
 export 'source.dart';
 /**
  * Instances of the class `FileBasedSource` implement a source that represents a file.
  *
  * @coverage dart.engine.source
  */
 class FileBasedSource implements Source {
 
   /**
@@ skipping 313 matching lines @@
     }
     return null;
   }
   Source resolveAbsolute(ContentCache contentCache, Uri uri) {
     if (!isFileUri(uri)) {
       return null;
     }
     return new FileBasedSource.con1(contentCache, new JavaFile.fromUri(uri));
   }
 }