Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect

Index: pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect b/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
index ed5fd1f40f045c525820711ee5ddd50c3c8e94d4..d7371d50823cbe58c162f795b18fd8422439ad3f 100644
--- a/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
@@ -3,11 +3,11 @@ import self as self;
 import "dart:core" as core;
 
 class Base extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Mixin extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method foo() → dynamic
     ;
@@ -15,7 +15,7 @@ class Mixin extends core::Object {
 abstract class Base&Mixin = self::Base with self::Mixin {
 }
 class Sub extends self::Base&Mixin {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static method main() → dynamic