Both unit and library sources must be isInSystemLibrary.

pkg/analyzer/lib/src/summary/summary_sdk.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.summary.summary_sdk;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/context/cache.dart' show CacheEntry;
 import 'package:analyzer/src/context/context.dart';
@@ skipping 32 matching lines @@
   bool compute(CacheEntry entry, ResultDescriptor result) {
     if (result == TYPE_PROVIDER) {
       entry.setValue(result, typeProvider, TargetedResult.EMPTY_LIST);
       return true;
     }
     AnalysisTarget target = entry.target;
     // Only SDK sources after this point.
     if (target.source == null || !target.source.isInSystemLibrary) {
       return false;
     }
-//    print('SummarySdkAnalysisContext: $result of $target');
     // Constant expressions are always resolved in summaries.
     if (result == CONSTANT_EXPRESSION_RESOLVED &&
         target is ConstantEvaluationTarget) {
       entry.setValue(result, true, TargetedResult.EMPTY_LIST);
       return true;
     }
     if (target is Source) {
       if (result == LIBRARY_ELEMENT1 ||
           result == LIBRARY_ELEMENT2 ||
           result == LIBRARY_ELEMENT3 ||
@@ skipping 20 matching lines @@
         if (kind != null) {
           entry.setValue(result, kind, TargetedResult.EMPTY_LIST);
           return true;
         }
         return false;
       } else {
 //        throw new UnimplementedError('$result of $target');
       }
     }
     if (target is LibrarySpecificUnit) {
+      if (target.library == null || !target.library.isInSystemLibrary) {
+        return false;
+      }
       if (result == COMPILATION_UNIT_ELEMENT) {
-        String uri1 = target.library.uri.toString();
-        String uri2 = target.unit.uri.toString();
-        CompilationUnitElement unit = resynthesizer
-            .getElement(new ElementLocationImpl.con3(<String>[uri1, uri2]));
+        String libraryUri = target.library.uri.toString();
+        String unitUri = target.unit.uri.toString();
+        CompilationUnitElement unit = resynthesizer.getElement(
+            new ElementLocationImpl.con3(<String>[libraryUri, unitUri]));
         if (unit != null) {
           entry.setValue(result, unit, TargetedResult.EMPTY_LIST);
           return true;
         }
       }
     }
     return false;
   }
 
   void _buildAsyncLibrary() {
@@ skipping 295 matching lines @@
    * throw a [StateError] if there is no class with the given name.
    */
   InterfaceType _getType(LibraryElement library, String name) {
     Element element = library.getType(name);
     if (element == null) {
       throw new StateError("No definition of type $name");
     }
     return (element as ClassElement).type;
   }
 }