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

pkg/analyzer/test/generated/resolver_test.dart

Index: pkg/analyzer/test/generated/resolver_test.dart
diff --git a/pkg/analyzer/test/generated/resolver_test.dart b/pkg/analyzer/test/generated/resolver_test.dart
index 102956ae860a7fdacc0d565196d82cb2b2a17888..e3756ac9abb873dec40b31ce4c576e8b180eb447 100644
--- a/pkg/analyzer/test/generated/resolver_test.dart
+++ b/pkg/analyzer/test/generated/resolver_test.dart
@@ -2920,8 +2920,12 @@ class TypeProviderImplTest extends EngineTestCase {
         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);
 
     //