Properly flatten FutureOr types when inferring the type of an async method.

pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
index 09cc16385b2ca1ce0666d6b15011ef484cf10dcc..4a814353195cbb36994b1512ca9d8fc3835c607b 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
@@ -60,9 +60,8 @@ class ClosureContext {
             returnContext, inferrer.coreTypes.iterableClass);
       }
     } else if (isAsync) {
-      returnContext = inferrer.wrapType(
-          inferrer.typeSchemaEnvironment.flattenFutures(returnContext),
-          inferrer.coreTypes.futureOrClass);
+      returnContext = inferrer.wrapFutureOrType(
+          inferrer.typeSchemaEnvironment.flattenFutures(returnContext));
     }
     return new ClosureContext._(isAsync, isGenerator, returnContext);
   }
@@ -532,7 +531,8 @@ abstract class TypeInferrerImpl extends TypeInferrer {
     }
     // TODO(paulberry): If [type] is a subtype of `Future`, should we just
     // return it unmodified?
-    return new InterfaceType(coreTypes.futureOrClass, <DartType>[type]);
+    return new InterfaceType(
+        coreTypes.futureOrClass, <DartType>[type ?? const DynamicType()]);
   }
 
   DartType wrapFutureType(DartType type) {