Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_field_overrides_getter.dart.outline.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 abstract class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   abstract get x() → core::int;
 }
 class B extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   get x() → core::int
     ;
 }
 class C extends self::A {
   field dynamic x;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class D extends self::B {
   field dynamic x;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class E extends core::Object implements self::A {
   field dynamic x;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class F extends core::Object implements self::B {
   field dynamic x;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 abstract class Object&B = core::Object with self::B {
 }
 class G extends self::Object&B {
   field dynamic x;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static method main() → dynamic
   ;