Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class Test1 extends core::Object {
   field core::int t = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method test() → void {
     dynamic v1 = this.t = self::getInt();
     dynamic v2 = this.t = self::getNum();
     dynamic v4 = let final dynamic #t1 = this.t in #t1.==(null) ? this.t = self::getInt() : #t1;
     dynamic v5 = let final dynamic #t2 = this.t in #t2.==(null) ? this.t = self::getNum() : #t2;
     dynamic v7 = this.t = this.t.+(self::getInt());
     dynamic v8 = this.t = this.t.+(self::getNum());
     dynamic v10 = this.t = this.t.+(1);
     dynamic v11 = let final dynamic #t3 = this.t in let final dynamic #t4 = this.t = #t3.+(1) in #t3;
   }
 }
 class Test2 extends core::Object {
   field core::num t = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method test() → void {
     dynamic v1 = this.t = self::getInt();
     dynamic v2 = this.t = self::getNum();
     dynamic v3 = this.t = self::getDouble();
     dynamic v4 = let final dynamic #t5 = this.t in #t5.==(null) ? this.t = self::getInt() : #t5;
     dynamic v5 = let final dynamic #t6 = this.t in #t6.==(null) ? this.t = self::getNum() : #t6;
     dynamic v6 = let final dynamic #t7 = this.t in #t7.==(null) ? this.t = self::getDouble() : #t7;
     dynamic v7 = this.t = this.t.+(self::getInt());
     dynamic v8 = this.t = this.t.+(self::getNum());
     dynamic v9 = this.t = this.t.+(self::getDouble());
     dynamic v10 = this.t = this.t.+(1);
     dynamic v11 = let final dynamic #t8 = this.t in let final dynamic #t9 = this.t = #t8.+(1) in #t8;
   }
 }
 class Test3 extends core::Object {
   field core::double t = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method test3() → void {
     dynamic v2 = this.t = self::getNum();
     dynamic v3 = this.t = self::getDouble();
     dynamic v5 = let final dynamic #t10 = this.t in #t10.==(null) ? this.t = self::getNum() : #t10;
     dynamic v6 = let final dynamic #t11 = this.t in #t11.==(null) ? this.t = self::getDouble() : #t11;
     dynamic v7 = this.t = this.t.+(self::getInt());
     dynamic v8 = this.t = this.t.+(self::getNum());
     dynamic v9 = this.t = this.t.+(self::getDouble());
     dynamic v10 = this.t = this.t.+(1);
     dynamic v11 = let final dynamic #t12 = this.t in let final dynamic #t13 = this.t = #t12.+(1) in #t12;
   }
 }
 static method getInt() → core::int
   return 0;
 static method getNum() → core::num
   return 0;
 static method getDouble() → core::double
   return 0.0;
 static method main() → dynamic {}