[dart2js_runtime_metrics] Update pubspec to 3.8

Change-Id: I44b084d41bf79ebebd8170bb5ebab1a4e3c2c25b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/427561
Auto-Submit: Mayank Patke <[email protected]>
Reviewed-by: Nate Biggs <[email protected]>
Commit-Queue: Nate Biggs <[email protected]>

Author: fishythefish

pkg/dart2js_runtime_metrics/pubspec.yaml

@@ -7,7 +7,7 @@ description: >-
   This library provides access to the measurements at runtime.
 
 environment:
-  sdk: "^3.7.0"
+  sdk: "^3.8.0"
 
 resolution: workspace