Revert "[dart2js] Exporting as package: to allow non-SDK usage"

This reverts commit 8aeeffb84825fa91c8b270afe2cdf77fad1cf1d5.

Reason for revert: Bad fix. Doesn't resolve underlying bug

Original change's description:
> [dart2js] Exporting as package: to allow non-SDK usage
>
> Change-Id: I0235f524904a1a4c54c83161eb51ab0277473d1e
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/244180
> Reviewed-by: Joshua Litt <joshualitt@google.com>
> Commit-Queue: Mark Zhou <markzipan@google.com>

TBR=markzipan@google.com,joshualitt@google.com

Change-Id: I0e2f8ff70c86ff1e69608f715fa5545419e63314
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/244182
Reviewed-by: Mark Zhou <markzipan@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Auto-Submit: Mark Zhou <markzipan@google.com>
diff --git a/pkg/dart2js_runtime_metrics/lib/_runtime_metrics_dart2js.dart b/pkg/dart2js_runtime_metrics/lib/_runtime_metrics_dart2js.dart
index ab66483..3aa7cbb 100644
--- a/pkg/dart2js_runtime_metrics/lib/_runtime_metrics_dart2js.dart
+++ b/pkg/dart2js_runtime_metrics/lib/_runtime_metrics_dart2js.dart
@@ -2,5 +2,4 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-export 'package:dart2js_runtime_metrics/runtime_metrics.dart'
-    show runtimeMetrics;
+export 'dart:_dart2js_runtime_metrics' show runtimeMetrics;
diff --git a/pkg/dart2js_runtime_metrics/lib/_startup_metrics_dart2js.dart b/pkg/dart2js_runtime_metrics/lib/_startup_metrics_dart2js.dart
index 424ec11..021c666 100644
--- a/pkg/dart2js_runtime_metrics/lib/_startup_metrics_dart2js.dart
+++ b/pkg/dart2js_runtime_metrics/lib/_startup_metrics_dart2js.dart
@@ -2,5 +2,4 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-export 'package:dart2js_runtime_metrics/startup_metrics.dart'
-    show startupMetrics;
+export 'dart:_dart2js_runtime_metrics' show startupMetrics;