Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000068.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 import "package:expect/expect.dart" as exp;
 
 class G<T extends core::Object> 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::•()
     ;
 }
 class C extends self::B {
-  constructor •() → void
+  default constructor •() → void
     : super self::B::•()
     ;
 }
 static method main() → dynamic {
   exp::Expect::isFalse(new self::G::•<self::B>() is self::G<self::C>);
   exp::Expect::isFalse(new self::G::•<self::A>() is self::G<self::B>);
   exp::Expect::isFalse(new self::G::•<self::A>() is self::G<self::C>);
   exp::Expect::isFalse(new self::G::•<core::Object>() is self::G<self::B>);
   exp::Expect::isFalse(new self::G::•<core::int>() is self::G<self::B>);
   exp::Expect::isFalse(new self::G::•<core::int>() is self::G<core::double>);
   exp::Expect::isFalse(new self::G::•<core::int>() is self::G<core::String>);
 }