Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_assign_to_index_set_vs_get.dart.strong.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class Index extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   operator +(self::F v) → self::C
     return null;
   operator -(core::int i) → self::C
     return null;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
   operator +(self::E v) → self::D
     return null;
   operator -(core::int i) → self::D
     return null;
 }
 class C extends self::B {
-  constructor •() → void
+  default constructor •() → void
     : super self::B::•()
     ;
 }
 class D extends self::C {
-  constructor •() → void
+  default constructor •() → void
     : super self::C::•()
     ;
 }
 class E extends self::D {
-  constructor •() → void
+  default constructor •() → void
     : super self::D::•()
     ;
 }
 class F extends self::E {
-  constructor •() → void
+  default constructor •() → void
     : super self::E::•()
     ;
 }
 class Test extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   operator [](self::Index i) → self::B
     return null;
   operator []=(self::Index i, self::A v) → void {}
   method test() → void {
     self::Test t = self::f<self::Test>();
     t.{self::Test::[]=}(self::f<dynamic>(), self::f<self::A>());
     let final dynamic #t1 = t in let final dynamic #t2 = self::f<dynamic>() in #t1.{self::Test::[]}(#t2).{core::Object::==}(null) ? let final dynamic #t3 = self::f<self::A>() in let final dynamic #t4 = #t1.{self::Test::[]=}(#t2, #t3) in #t3 : null;
     let final dynamic #t5 = t in let final dynamic #t6 = self::f<dynamic>() in #t5.{self::Test::[]=}(#t6, #t5.{self::Test::[]}(#t6).{self::A::+}(self::f<dynamic>()));
     let final dynamic #t7 = t in let final dynamic #t8 = self::f<dynamic>() in let final dynamic #t9 = #t7.{self::Test::[]}(#t8).{self::A::-}(1) in let final dynamic #t10 = #t7.{self::Test::[]=}(#t8, #t9) in #t9;
     let final dynamic #t11 = t in let final dynamic #t12 = self::f<dynamic>() in #t11.{self::Test::[]=}(#t12, #t11.{self::Test::[]}(#t12).{self::A::-}(1));
     self::A v1 = let final dynamic #t13 = t in let final dynamic #t14 = self::f<dynamic>() in let final dynamic #t15 = self::f<self::A>() in let final dynamic #t16 = #t13.{self::Test::[]=}(#t14, #t15) in #t15;
     self::A v2 = let final dynamic #t17 = t in let final dynamic #t18 = self::f<dynamic>() in let final dynamic #t19 = #t17.{self::Test::[]}(#t18) in #t19.{core::Object::==}(null) ? let final dynamic #t20 = self::f<self::A>() in let final dynamic #t21 = #t17.{self::Test::[]=}(#t18, #t20) in #t20 : #t19;
     self::C v3 = let final dynamic #t22 = t in let final dynamic #t23 = self::f<dynamic>() in let final dynamic #t24 = #t22.{self::Test::[]}(#t23).{self::A::+}(self::f<dynamic>()) in let final dynamic #t25 = #t22.{self::Test::[]=}(#t23, #t24) in #t24;
     self::C v4 = let final dynamic #t26 = t in let final dynamic #t27 = self::f<dynamic>() in let final dynamic #t28 = #t26.{self::Test::[]}(#t27).{self::A::-}(1) in let final dynamic #t29 = #t26.{self::Test::[]=}(#t27, #t28) in #t28;
     self::A v5 = let final dynamic #t30 = t in let final dynamic #t31 = self::f<dynamic>() in let final dynamic #t32 = #t30.{self::Test::[]}(#t31) in let final dynamic #t33 = #t30.{self::Test::[]=}(#t31, #t32.{self::A::-}(1)) in #t32;
   }
 }
 static method f<T extends core::Object>() → self::f::T
   return null;
 static method main() → dynamic {}