Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.strong.expect b/pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.strong.expect
index b85ee838bcf91dfb73e194bda39e094d78ac78ae..202125b3ee5d0b6b83f3a6c4a0a2030d9b1b94c9 100644
--- a/pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.strong.expect
+++ b/pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.strong.expect
@@ -3,17 +3,17 @@ 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
@@ -26,12 +26,12 @@ class B extends self::A {
     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