Allow SDKs to be analyzed that lack a definition of FutureOr<T>.

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/file_system/file_system.dart' show ResourceProvider;
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/dart/element/type.dart';
 import 'package:analyzer/src/generated/constant.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/resolver.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/source.dart'
     show DartUriResolver, Source, SourceFactory;
+import 'package:analyzer/src/generated/testing/element_factory.dart';
 import 'package:analyzer/src/summary/idl.dart';
 import 'package:analyzer/src/summary/package_bundle_reader.dart';
 
 /**
  * An implementation of [DartSdk] which provides analysis results for `dart:`
  * libraries from the given summary file.  This implementation is limited and
  * suitable only for command-line tools, but not for IDEs - it does not
  * implement [sdkLibraries], [sdkVersion], [uris] and [fromFileUri].
  */
 class SummaryBasedDartSdk implements DartSdk {
@@ skipping 157 matching lines @@
   @override
   InterfaceType get futureNullType {
     assert(_asyncLibrary != null);
     _futureNullType ??= futureType.instantiate(<DartType>[nullType]);
     return _futureNullType;
   }
 
   @override
   InterfaceType get futureOrType {
     assert(_asyncLibrary != null);
-    _futureOrType ??= _getType(_asyncLibrary, "FutureOr");
+    try {
+      _futureOrType ??= _getType(_asyncLibrary, "FutureOr");
+    } on StateError {
+      // FutureOr<T> is still fairly new, so if we're analyzing an SDK that
+      // doesn't have it yet, create an element for it.
+      _futureOrType =
+          TypeProviderImpl.createPlaceholderFutureOr(futureType, objectType);
+    }
     return _futureOrType;
   }
 
   @override
   InterfaceType get futureType {
     assert(_asyncLibrary != null);
     _futureType ??= _getType(_asyncLibrary, "Future");
     return _futureType;
   }
 
@@ skipping 129 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;
   }
 }