Add pubspec.yaml to runtime/tools

Otherwise Dart scripts there inherit the version from the root
pubspec, which can be ahead of prebuilt SDK.

Change-Id: I07b6672f72c08d73661fda4e333cbc2be767db52
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/440480
Reviewed-by: Alexander Thomas <[email protected]>
Auto-Submit: Ivan Inozemtsev <[email protected]>
Reviewed-by: Alexander Markov <[email protected]>
Commit-Queue: Ivan Inozemtsev <[email protected]>
Commit-Queue: Alexander Thomas <[email protected]>

Author: iinozemtsev

pubspec.yaml

@@ -77,6 +77,7 @@ workspace:
   - pkg/vm_snapshot_analysis
   - pkg/wasm_builder
   - runtime/tests/vm/dart
+  - runtime/tools
   - runtime/tools/profiling
   - samples/ffi/http
   - samples/ffi/httpIG

runtime/tools/pubspec.yaml

@@ -0,0 +1,8 @@
+name: runtime_tools
+description: Internal runtime tools.
+
+environment:
+  sdk: ^3.9.0
+
+resolution: workspace
+publish_to: none