Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000068.dart.outline.expect

Index: pkg/front_end/testcases/rasta/issue_000068.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000068.dart.outline.expect b/pkg/front_end/testcases/rasta/issue_000068.dart.outline.expect
index af7d585acb9ac0cd1d333a897249f7e31204c6d2..08bd936a4e36ac6dd8b6722eb4069a5c8c3a30e0 100644
--- a/pkg/front_end/testcases/rasta/issue_000068.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/issue_000068.dart.outline.expect
@@ -3,19 +3,19 @@ import self as self;
 import "dart:core" as core;
 
 class G<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class C extends self::B {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static method main() → dynamic