Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/override.dart.outline.expect
diff --git a/pkg/front_end/testcases/override.dart.outline.expect b/pkg/front_end/testcases/override.dart.outline.expect
index 17cdbfcd0704fd21a2929277c88b48cb2f79c452..2cd93ccc5d79d42b864855960648a585dcfa177c 100644
--- a/pkg/front_end/testcases/override.dart.outline.expect
+++ b/pkg/front_end/testcases/override.dart.outline.expect
@@ -3,21 +3,21 @@ import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Bar extends self::Foo {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Base extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method method() → self::Foo
     ;
 }
 class Sub extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     ;
   method method() → self::Foo
     ;