Add access to FutureOr in TypeProviders (and add implementation in mock sdks)

pkg/analyzer/lib/src/summary/summary_sdk.dart

Index: pkg/analyzer/lib/src/summary/summary_sdk.dart
diff --git a/pkg/analyzer/lib/src/summary/summary_sdk.dart b/pkg/analyzer/lib/src/summary/summary_sdk.dart
index 3142b291e16cb47b47879b719437d85e9af83d2f..ac6d4b3e0e3370c7ca75204fdb27e94641479244 100644
--- a/pkg/analyzer/lib/src/summary/summary_sdk.dart
+++ b/pkg/analyzer/lib/src/summary/summary_sdk.dart
@@ -200,6 +200,7 @@ class SummaryTypeProvider extends TypeProviderBase {
   InterfaceType _functionType;
   InterfaceType _futureDynamicType;
   InterfaceType _futureNullType;
+  InterfaceType _futureOrType;
   InterfaceType _futureType;
   InterfaceType _intType;
   InterfaceType _iterableDynamicType;
@@ -266,6 +267,13 @@ class SummaryTypeProvider extends TypeProviderBase {
   }
 
   @override
+  InterfaceType get futureOrType {
+    assert(_asyncLibrary != null);
+    _futureOrType ??= _getType(_asyncLibrary, "FutureOr");
+    return _futureOrType;
+  }
+
+  @override
   InterfaceType get futureType {
     assert(_asyncLibrary != null);
     _futureType ??= _getType(_asyncLibrary, "Future");