Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
   operator +(self::C v) → self::A
     return null;
   operator -(core::int i) → self::B
     return null;
   operator *(self::B v) → self::B
     return null;
   operator &(self::A v) → self::C
     return null;
 }
 class C extends self::B {
-  constructor •() → void
+  default constructor •() → void
     : super self::B::•()
     ;
 }
 class Test extends core::Object {
   field self::B member = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method test() → void {
     this.member = self::f<dynamic>();
     this.member.==(null) ? this.member = self::f<dynamic>() : null;
     this.member = this.member.+(self::f<dynamic>());
     this.member = this.member.*(self::f<dynamic>());
     this.member = this.member.&(self::f<dynamic>());
     this.member = this.member.-(1);
     this.member = this.member.-(1);
     dynamic v1 = this.member = self::f<dynamic>();
     dynamic v2 = let final dynamic #t1 = this.member in #t1.==(null) ? this.member = self::f<dynamic>() : #t1;
     dynamic v3 = this.member = this.member.+(self::f<dynamic>());
     dynamic v4 = this.member = this.member.*(self::f<dynamic>());
     dynamic v5 = this.member = this.member.&(self::f<dynamic>());
     dynamic v6 = this.member = this.member.-(1);
     dynamic v7 = let final dynamic #t2 = this.member in let final dynamic #t3 = this.member = #t2.-(1) in #t2;
   }
 }
 static method f<T extends core::Object>() → self::f::T
   return null;
 static method main() → dynamic {}