Handle FutureOr type when inferring async closures.

pkg/kernel/lib/core_types.dart

Index: pkg/kernel/lib/core_types.dart
diff --git a/pkg/kernel/lib/core_types.dart b/pkg/kernel/lib/core_types.dart
index 2d446fa9c2966356a7891272dbac571effffdb4b..dada2ee0d7cf17af1d9c12b94dc905aefd5c60de 100644
--- a/pkg/kernel/lib/core_types.dart
+++ b/pkg/kernel/lib/core_types.dart
@@ -20,6 +20,7 @@ class CoreTypes extends LibraryIndex {
   Class iterableClass;
   Class iteratorClass;
   Class futureClass;
+  Class futureOrClass;
   Class streamClass;
   Class symbolClass;
   Class internalSymbolClass;
@@ -71,6 +72,7 @@ class CoreTypes extends LibraryIndex {
     functionClass = getClass('dart:core', 'Function');
     invocationClass = getClass('dart:core', 'Invocation');
     futureClass = getClass('dart:async', 'Future');
+    futureOrClass = getClass('dart:async', 'FutureOr');
     streamClass = getClass('dart:async', 'Stream');
     internalSymbolClass = getClass('dart:_internal', 'Symbol');
   }