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

pkg/analyzer/test/src/context/mock_sdk.dart

Index: pkg/analyzer/test/src/context/mock_sdk.dart
diff --git a/pkg/analyzer/test/src/context/mock_sdk.dart b/pkg/analyzer/test/src/context/mock_sdk.dart
index 03f0af30f3b98e8ffa7c05e3a4243bd11613df0c..8c796a9a10dcbf7adfb0ece034efa4781d50291d 100644
--- a/pkg/analyzer/test/src/context/mock_sdk.dart
+++ b/pkg/analyzer/test/src/context/mock_sdk.dart
@@ -52,6 +52,8 @@ class Future<T> {
   Future<T> whenComplete(action());
 }
 
+class FutureOr<T> {}
+
 abstract class Completer<T> {
   factory Completer() => new _AsyncCompleter<T>();
   factory Completer.sync() => new _SyncCompleter<T>();