Tolerate SDKs that do not include dart:async (issue 25520)

pkg/analyzer/lib/src/task/dart.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.task.dart;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 1560 matching lines @@
   BuildTypeProviderTask(
       InternalAnalysisContext context, AnalysisContextTarget target)
       : super(context, target);
 
   @override
   TaskDescriptor get descriptor => DESCRIPTOR;
 
   @override
   void internalPerform() {
     LibraryElement coreLibrary = getRequiredInput(CORE_INPUT);
-    LibraryElement asyncLibrary = getRequiredInput(ASYNC_INPUT);
+    LibraryElement asyncLibrary = getOptionalInput(ASYNC_INPUT);
+    if (asyncLibrary == null) {
+      asyncLibrary =
+          (context as AnalysisContextImpl).createMockAsyncLib(coreLibrary);
+    }
     Namespace coreNamespace = coreLibrary.publicNamespace;
     Namespace asyncNamespace = asyncLibrary.publicNamespace;
     //
     // Record outputs.
     //
     if (!context.analysisOptions.enableAsync) {
       AnalysisContextImpl contextImpl = context;
       asyncLibrary = contextImpl.createMockAsyncLib(coreLibrary);
       asyncNamespace = asyncLibrary.publicNamespace;
     }
     TypeProvider typeProvider =
         new TypeProviderImpl.forNamespaces(coreNamespace, asyncNamespace);
     (context as InternalAnalysisContext).typeProvider = typeProvider;
     outputs[TYPE_PROVIDER] = typeProvider;
   }
 
   static Map<String, TaskInput> buildInputs(AnalysisTarget target) {
     AnalysisContextTarget contextTarget = target;
     SourceFactory sourceFactory = contextTarget.context.sourceFactory;
     Source coreSource = sourceFactory.forUri(DartSdk.DART_CORE);
     Source asyncSource = sourceFactory.forUri(DartSdk.DART_ASYNC);
+    if (asyncSource == null) {
+      return <String, TaskInput>{CORE_INPUT: LIBRARY_ELEMENT3.of(coreSource)};
+    }
     return <String, TaskInput>{
       CORE_INPUT: LIBRARY_ELEMENT3.of(coreSource),
       ASYNC_INPUT: LIBRARY_ELEMENT3.of(asyncSource)
     };
   }
 
   /**
    * Create a [BuildTypeProviderTask] based on the given [context].
    */
   static BuildTypeProviderTask createTask(
@@ skipping 3770 matching lines @@
 
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.removeLast();
     return true;
   }
 }