Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.direct.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::•()
     ;
 }
 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 {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   operator [](self::Index i) → self::B
     return null;
   operator []=(self::Index i, self::B v) → void {}
   method test() → void {
     this.[]=(self::f<dynamic>(), self::f<dynamic>());
     let final dynamic #t1 = self::f<dynamic>() in this.[](#t1).==(null) ? let final dynamic #t2 = self::f<dynamic>() in let final dynamic #t3 = this.[]=(#t1, #t2) in #t2 : null;
     let final dynamic #t4 = self::f<dynamic>() in this.[]=(#t4, this.[](#t4).+(self::f<dynamic>()));
     let final dynamic #t5 = self::f<dynamic>() in this.[]=(#t5, this.[](#t5).*(self::f<dynamic>()));
     let final dynamic #t6 = self::f<dynamic>() in this.[]=(#t6, this.[](#t6).&(self::f<dynamic>()));
     let final dynamic #t7 = self::f<dynamic>() in let final dynamic #t8 = this.[](#t7).-(1) in let final dynamic #t9 = this.[]=(#t7, #t8) in #t8;
     let final dynamic #t10 = self::f<dynamic>() in this.[]=(#t10, this.[](#t10).-(1));
     dynamic v1 = let final dynamic #t11 = self::f<dynamic>() in let final dynamic #t12 = self::f<dynamic>() in let final dynamic #t13 = this.[]=(#t11, #t12) in #t12;
     dynamic v2 = let final dynamic #t14 = self::f<dynamic>() in let final dynamic #t15 = this.[](#t14) in #t15.==(null) ? let final dynamic #t16 = self::f<dynamic>() in let final dynamic #t17 = this.[]=(#t14, #t16) in #t16 : #t15;
     dynamic v3 = let final dynamic #t18 = self::f<dynamic>() in let final dynamic #t19 = this.[](#t18).+(self::f<dynamic>()) in let final dynamic #t20 = this.[]=(#t18, #t19) in #t19;
     dynamic v4 = let final dynamic #t21 = self::f<dynamic>() in let final dynamic #t22 = this.[](#t21).*(self::f<dynamic>()) in let final dynamic #t23 = this.[]=(#t21, #t22) in #t22;
     dynamic v5 = let final dynamic #t24 = self::f<dynamic>() in let final dynamic #t25 = this.[](#t24).&(self::f<dynamic>()) in let final dynamic #t26 = this.[]=(#t24, #t25) in #t25;
     dynamic v6 = let final dynamic #t27 = self::f<dynamic>() in let final dynamic #t28 = this.[](#t27).-(1) in let final dynamic #t29 = this.[]=(#t27, #t28) in #t28;
     dynamic v7 = let final dynamic #t30 = self::f<dynamic>() in let final dynamic #t31 = this.[](#t30) in let final dynamic #t32 = this.[]=(#t30, #t31.-(1)) in #t31;
   }
 }
 static method f<T extends core::Object>() → self::f::T
   return null;
 static method main() → dynamic {}