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

pkg/analyzer/lib/src/task/dart.dart

Index: pkg/analyzer/lib/src/task/dart.dart
diff --git a/pkg/analyzer/lib/src/task/dart.dart b/pkg/analyzer/lib/src/task/dart.dart
index b52bed60c9c576c0ef6e1e9eb84ff2de88b3e299..e99e82da669f4dbad2e0add88b3bec3cc9d874ef 100644
--- a/pkg/analyzer/lib/src/task/dart.dart
+++ b/pkg/analyzer/lib/src/task/dart.dart
@@ -1578,7 +1578,11 @@ class BuildTypeProviderTask extends SourceBasedAnalysisTask {
   @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;
     //
@@ -1600,6 +1604,9 @@ class BuildTypeProviderTask extends SourceBasedAnalysisTask {
     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)