[dart2js] Flip new deferred split flag.

Change-Id: Ifa795b96f2d2fa685072f314508b761b77a39bf2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/160161
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Joshua Litt <joshualitt@google.com>

CHANGELOG.md

@@ -31,6 +31,11 @@
     unsoundness with only a minimal regression. To explicitly disable
     deferred loading of types, pass `--no-defer-class-types`. See the original
     post on the [unsoundness in the deferred loading algorithm][].
+*   Enables a new sound deferred splitting algorithm. To explicitly disable
+    the new deferred splitting algorithm, pass `--no-new-deferred-split'.
+    See the original post on the
+    [unsoundness in the deferred loading algorithm][].
+
 
 [#42982]: https://github.com/dart-lang/sdk/issues/42982
 [unsoundness in the deferred loading algorithm]: https://github.com/dart-lang/sdk/blob/302ad7ab2cd2de936254850550aad128ae76bbb7/CHANGELOG.md#dart2js-3

pkg/compiler/lib/src/options.dart

@@ -146,7 +146,7 @@ class CompilerOptions implements DiagnosticOptions {
   /// When [reportInvalidInferredDeferredTypes] shows no errors, we expect this
   /// flag to produce the same or better results than the current unsound
   /// implementation.
-  bool newDeferredSplit = false; // default value.
+  bool newDeferredSplit = true; // default value.
   bool _newDeferredSplit = false;
   bool _noNewDeferredSplit = false;
 

pkg/compiler/test/deferred_loading/data/regress_43055/libb.dart

@@ -6,6 +6,6 @@
 
 import 'libc.dart';
 
-/*class: C1:OutputUnit(1, {libb}), type=OutputUnit(1, {libb})*/
+/*class: C1:OutputUnit(1, {libb}), type=OutputUnit(main, {})*/
 /*member: C1.:OutputUnit(1, {libb})*/
 class C1 extends C2 implements C3 {}