Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/micro.dart.strong.expect

Index: pkg/front_end/testcases/micro.dart.strong.expect
diff --git a/pkg/front_end/testcases/micro.dart.strong.expect b/pkg/front_end/testcases/micro.dart.strong.expect
index fdc6c950508a256540cd5936456d310feccecab5..8c084282e27236b1c786803055869a35e7c8e739 100644
--- a/pkg/front_end/testcases/micro.dart.strong.expect
+++ b/pkg/front_end/testcases/micro.dart.strong.expect
@@ -3,7 +3,7 @@ import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method instanceMethod() → dynamic {
@@ -11,19 +11,19 @@ class Foo extends core::Object {
   }
 }
 abstract class ExternalValue extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 abstract class Bar extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   abstract method externalInstanceMethod() → self::ExternalValue;
 }
 class Box extends core::Object {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
@@ -39,13 +39,13 @@ class SubFinalBox extends self::FinalBox {
     ;
 }
 class DynamicReceiver1 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method dynamicallyCalled(dynamic x) → dynamic {}
 }
 class DynamicReceiver2 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method dynamicallyCalled(dynamic x) → dynamic {}