Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000070.dart.outline.expect

Index: pkg/front_end/testcases/rasta/issue_000070.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000070.dart.outline.expect b/pkg/front_end/testcases/rasta/issue_000070.dart.outline.expect
index 455f514611ff7f94b1d972930385183f4850079a..1553d9c44f3ffc14448647b229458188d729f3c5 100644
--- a/pkg/front_end/testcases/rasta/issue_000070.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/issue_000070.dart.outline.expect
@@ -18,11 +18,11 @@ class A<N extends core::Object, S extends core::Object, U extends core::Object>
     ;
 }
 abstract class J<Aa extends core::Object, B extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 abstract class I<H extends core::Object, C extends core::Object, K extends core::Object> extends self::J<self::I::C, self::I::K> {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static method main() → dynamic