Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000080.dart.outline.expect

Index: pkg/front_end/testcases/rasta/issue_000080.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000080.dart.outline.expect b/pkg/front_end/testcases/rasta/issue_000080.dart.outline.expect
index 585c3d5e0bb2cda4715f7cf5bff084cd26d13b62..e9fe72d834eb9cf7d9d2a75b3b9f1b13bc6730d1 100644
--- a/pkg/front_end/testcases/rasta/issue_000080.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/issue_000080.dart.outline.expect
@@ -4,7 +4,7 @@ import "dart:core" as core;
 
 class Mixin extends core::Object {
   field dynamic field;
-  constructor •() → void
+  default constructor •() → void
     ;
   method foo() → dynamic
     ;
@@ -12,7 +12,7 @@ class Mixin extends core::Object {
 abstract class Object&Mixin = core::Object with self::Mixin {
 }
 class Foo extends self::Object&Mixin {
-  constructor •() → void
+  default constructor •() → void
     ;
   method foo() → dynamic
     ;