Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/prefer_baseclass.dart.outline.expect

Index: pkg/front_end/testcases/prefer_baseclass.dart.outline.expect
diff --git a/pkg/front_end/testcases/prefer_baseclass.dart.outline.expect b/pkg/front_end/testcases/prefer_baseclass.dart.outline.expect
index a5009b2fda614f718358849d5ac236466d479eaf..01d805fc31a27cc87a9eb67b72d549dd875af312 100644
--- a/pkg/front_end/testcases/prefer_baseclass.dart.outline.expect
+++ b/pkg/front_end/testcases/prefer_baseclass.dart.outline.expect
@@ -3,27 +3,27 @@ import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class B extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class AB1 extends self::A implements self::B {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class AB2 extends self::A implements self::B {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class BA1 extends self::B implements self::A {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class BA2 extends self::B implements self::A {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static method takeSubclassOfA(dynamic obj) → dynamic