Allow `return;` in async method.

pkg/compiler/lib/src/typechecker.dart

Index: pkg/compiler/lib/src/typechecker.dart
diff --git a/pkg/compiler/lib/src/typechecker.dart b/pkg/compiler/lib/src/typechecker.dart
index 82d829164ac9c90eddf7add4120202bd9f628c5c..3ad77a2f01f465ff83a4e1f70bcb0a0bd541c9c5 100644
--- a/pkg/compiler/lib/src/typechecker.dart
+++ b/pkg/compiler/lib/src/typechecker.dart
@@ -1610,7 +1610,8 @@ class TypeCheckerVisitor extends Visitor<DartType> {
           checkAssignable(expression, expressionType, expectedReturnType);
         }
       }
-
+    } else if (currentAsyncMarker == AsyncMarker.ASYNC) {
+      // `return;` is allowed.
     } else if (!types.isAssignable(expectedReturnType, const VoidType())) {
       // Let f be the function immediately enclosing a return statement of the
       // form 'return;' It is a static warning if both of the following