Fix restoring of async try context in catch clause (issue 23498).

tests/language/regress_23498_test.dart

Index: tests/language/regress_23498_test.dart
diff --git a/tests/language/regress_22579_test.dart b/tests/language/regress_23498_test.dart
similarity index 69%
copy from tests/language/regress_22579_test.dart
copy to tests/language/regress_23498_test.dart
index 76c8f42463fe87b21372d1f2da3a768cf0482fb6..ba7a1406d337b5611ea7076fe67760b87009909e 100644
--- a/tests/language/regress_22579_test.dart
+++ b/tests/language/regress_23498_test.dart
@@ -7,10 +7,16 @@ import "package:expect/expect.dart";
 
 foo() async {
   try {
-    await 1;
-  } catch(e) {
+    try {
+      await new Future.error('error');
+    } catch (error) {
+      print("caught once");
+      throw 'error';
+    }
+  } catch (error) {
+    print("caught twice");
+    throw 'error';
   }
-  throw "error";
 }
 
 main() async {