Avoid warning when using `return;` from sync* and async* functions.

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 323c19db73bf53503e3be49d82ec0bf79c3e23b1..6ca167a27f0193c6d9c913a2896aa6a2e424a1d3 100644
--- a/pkg/compiler/lib/src/typechecker.dart
+++ b/pkg/compiler/lib/src/typechecker.dart
@@ -1655,7 +1655,7 @@ class TypeCheckerVisitor extends Visitor<DartType> {
           checkAssignable(expression, expressionType, expectedReturnType);
         }
       }
-    } else if (currentAsyncMarker == AsyncMarker.ASYNC) {
+    } else if (currentAsyncMarker != AsyncMarker.SYNC) {
       // `return;` is allowed.
     } else if (!types.isAssignable(expectedReturnType, const VoidType())) {
       // Let f be the function immediately enclosing a return statement of the