Issue 26639. Set source for mock 'dart:async' library.

pkg/analyzer/test/generated/resolver_test.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.
 
 library analyzer.test.generated.resolver_test;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 2902 matching lines @@
       stackTraceType.element,
       stringType.element,
       symbolType.element,
       typeType.element
     ];
     AnalysisContext context = AnalysisEngine.instance.createAnalysisContext();
     LibraryElementImpl coreLibrary = new LibraryElementImpl.forNode(
         context, AstFactory.libraryIdentifier2(["dart.core"]));
     coreLibrary.definingCompilationUnit = coreUnit;
 
-    LibraryElementImpl mockAsyncLib =
-        (context as AnalysisContextImpl).createMockAsyncLib(coreLibrary);
+    Source asyncSource = new NonExistingSource(
+        'async.dart', Uri.parse('dart:async'), UriKind.DART_URI);
+    LibraryElementImpl mockAsyncLib = (context as AnalysisContextImpl)
+        .createMockAsyncLib(coreLibrary, asyncSource);
+    expect(mockAsyncLib.source, same(asyncSource));
+    expect(mockAsyncLib.definingCompilationUnit.source, same(asyncSource));
     expect(mockAsyncLib.publicNamespace, isNotNull);
 
     //
     // Create a type provider and ensure that it can return the expected types.
     //
     TypeProviderImpl provider = new TypeProviderImpl(coreLibrary, mockAsyncLib);
     expect(provider.boolType, same(boolType));
     expect(provider.bottomType, isNotNull);
     expect(provider.doubleType, same(doubleType));
     expect(provider.dynamicType, isNotNull);
@@ skipping 692 matching lines @@
  */
 class _StaleElement extends ElementImpl {
   _StaleElement() : super("_StaleElement", -1);
 
   @override
   get kind => throw "_StaleElement's kind shouldn't be accessed";
 
   @override
   accept(_) => throw "_StaleElement shouldn't be visited";
 }