Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/conflicts_can_happen2.dart.strong.expect

Index: pkg/front_end/testcases/inference/conflicts_can_happen2.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/conflicts_can_happen2.dart.strong.expect b/pkg/front_end/testcases/inference/conflicts_can_happen2.dart.strong.expect
index 55d90f99d85a739ebd8bbbf8f6800bb0d6591757..cc41ea36470ba7b25d21581d1d322ff186fcc52b 100644
--- a/pkg/front_end/testcases/inference/conflicts_can_happen2.dart.strong.expect
+++ b/pkg/front_end/testcases/inference/conflicts_can_happen2.dart.strong.expect
@@ -4,44 +4,44 @@ import "dart:core" as core;
 
 class I1 extends core::Object {
   field core::int x = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class I2 extends core::Object {
   field core::int y = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class I3 extends core::Object implements self::I1, self::I2 {
   field core::int x = null;
   field core::int y = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class A extends core::Object {
   final field self::I1 a = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends core::Object {
   final field self::I2 a = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class C1 extends core::Object implements self::A, self::B {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   get a() → self::I3
     return null;
 }
 class C2 extends core::Object implements self::A, self::B {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   get a() → dynamic