Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/escape.dart.direct.expect

Index: pkg/front_end/testcases/escape.dart.direct.expect
diff --git a/pkg/front_end/testcases/escape.dart.direct.expect b/pkg/front_end/testcases/escape.dart.direct.expect
index 23706906e37f8343f61ef2e7e63d32adeabff9f9..db642e8e5b8526e48f5a501d09a5f27462e16d78 100644
--- a/pkg/front_end/testcases/escape.dart.direct.expect
+++ b/pkg/front_end/testcases/escape.dart.direct.expect
@@ -4,18 +4,18 @@ import "dart:core" as core;
 
 class A extends core::Object {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends core::Object {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class C extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   operator ==(dynamic x) → dynamic
@@ -23,7 +23,7 @@ class C extends core::Object {
 }
 class X extends core::Object implements self::A, self::B {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }