Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/escape.dart.outline.expect

Index: pkg/front_end/testcases/escape.dart.outline.expect
diff --git a/pkg/front_end/testcases/escape.dart.outline.expect b/pkg/front_end/testcases/escape.dart.outline.expect
index 916979cee4dcdfcf1a25a1be8f09c1ff16b76e99..6b5cf2f1ca6d527583cf51ea680c9d3758cc3205 100644
--- a/pkg/front_end/testcases/escape.dart.outline.expect
+++ b/pkg/front_end/testcases/escape.dart.outline.expect
@@ -4,23 +4,23 @@ import "dart:core" as core;
 
 class A extends core::Object {
   field dynamic field;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class B extends core::Object {
   field dynamic field;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class C extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   operator ==(dynamic x) → dynamic
     ;
 }
 class X extends core::Object implements self::A, self::B {
   field dynamic field;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static method useAsA(self::A object) → void