Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/mixin.dart.outline.expect
diff --git a/pkg/front_end/testcases/mixin.dart.outline.expect b/pkg/front_end/testcases/mixin.dart.outline.expect
index 3dc6a7bf420ea6a441625893c19fee9575ebd048..3d909fc919aafb7c438513c9815df4c89c4650fc 100644
--- a/pkg/front_end/testcases/mixin.dart.outline.expect
+++ b/pkg/front_end/testcases/mixin.dart.outline.expect
@@ -11,13 +11,13 @@ class B extends self::Object&M1&M2 {
     ;
 }
 abstract class M1 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method m() → dynamic
     ;
 }
 abstract class M2 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method m() → dynamic
     ;
@@ -27,7 +27,7 @@ class C extends self::Object&M1&M2 {
     ;
 }
 abstract class G1<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method m() → dynamic
     ;
@@ -35,7 +35,7 @@ abstract class G1<T extends core::Object> extends core::Object {
 abstract class Object&G1^^#T0<#T0 extends core::Object> = core::Object with self::G1<self::Object&G1^^#T0::#T0> {
 }
 class D<S extends core::Object> extends self::Object&G1^^#T0<self::D::S> {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static method main() → dynamic