Issue 23027. Try to make priority files analyzable.

pkg/analyzer/lib/src/generated/engine.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.
 
 // 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;
 
 import "dart:math" as math;
@@ skipping 1087 matching lines @@
       // Failure to do so can result in an infinite loop in
       // performAnalysisTask() because re-caching one AST structure
       // can cause another priority source's AST structure to be flushed.
       //
       int count = math.min(
           sources.length, _options.cacheSize - _PRIORITY_ORDER_SIZE_DELTA);
       _priorityOrder = new List<Source>(count);
       for (int i = 0; i < count; i++) {
         _priorityOrder[i] = sources[i];
       }
+      // Ensure entries for every priority source.
+      for (var source in _priorityOrder) {
+        SourceEntry entry = _getReadableSourceEntry(source);
+        if (entry == null) {
+          _createSourceEntry(source, false);
+        }
+      }
     }
   }
 
   @override
   set contentCache(ContentCache value) {
     _contentCache = value;
   }
 
   @override
   DeclaredVariables get declaredVariables => _declaredVariables;
@@ skipping 10525 matching lines @@
   visitElement(Element element) {
     if (element.id == _id) {
       result = element;
       throw new _ElementByIdFinderException();
     }
     super.visitElement(element);
   }
 }
 
 class _ElementByIdFinderException {}