We need a CompilationUnit with element.library set.

pkg/analyzer/lib/src/context/context.dart

 // Copyright (c) 2015, 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.
 
 library analyzer.src.context.context;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1853 matching lines @@
         entry.setState(TOKEN_STREAM, CacheState.FLUSHED);
         entry.setState(PARSED_UNIT, CacheState.FLUSHED);
         SourceKind sourceKind = getKindOf(source);
         List<Source> partSources = getResult(source, INCLUDED_PARTS);
         if (sourceKind == SourceKind.LIBRARY && partSources.isEmpty) {
           Source librarySource = source;
           // Try to find an old unit which has element model.
           CacheEntry unitEntry =
               getCacheEntry(new LibrarySpecificUnit(librarySource, source));
           CompilationUnit oldUnit = RESOLVED_UNIT_RESULTS
+              .skipWhile((result) => result != RESOLVED_UNIT2)
               .map(unitEntry.getValue)
               .firstWhere((unit) => unit != null, orElse: () => null);
           // If we have the old unit, we can try to update it.
           if (oldUnit != null) {
             CompilationUnit newUnit = parseCompilationUnit(source);
             IncrementalCompilationUnitElementBuilder builder =
                 new IncrementalCompilationUnitElementBuilder(oldUnit, newUnit);
             builder.build();
             CompilationUnitElementDelta unitDelta = builder.unitDelta;
             if (!unitDelta.hasDirectiveChange) {
@@ skipping 411 matching lines @@
     }
     DartSdk sdk = factory.dartSdk;
     if (sdk == null) {
       throw new IllegalArgumentException(
           "The source factory for an SDK analysis context must have a DartUriResolver");
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }