Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     ;
   operator +(self::C v) → self::A
     ;
   operator -(core::int i) → self::B
     ;
   operator *(self::B v) → self::B
     ;
   operator &(self::A v) → self::C
     ;
 }
 class C extends self::B {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Test extends core::Object {
   field self::B member;
-  constructor •() → void
+  default constructor •() → void
     ;
   method test() → void
     ;
 }
 static method f<T extends core::Object>() → self::f::T
   ;
 static method main() → dynamic
   ;