Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/rasta/generic_factory.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/generic_factory.dart.outline.expect b/pkg/front_end/testcases/rasta/generic_factory.dart.outline.expect
index a02e017baa3248a373958cba12df36bc45d4103c..5da78bb877db5707a12b2a5e3d827a03bcefc2a6 100644
--- a/pkg/front_end/testcases/rasta/generic_factory.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/generic_factory.dart.outline.expect
@@ -3,15 +3,15 @@ import self as self;
 import "dart:core" as core;
 
 class C1 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class C2 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class C3 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class A<T extends core::Object> extends core::Object {