[ci] Add LUCI version of dart unit tests (flutter#4265)

Add initial LUCI versions of the dart unit tests task.

Part of flutter#114373

Author: stuartmorgan-g

.ci.yaml

@@ -92,6 +92,50 @@ targets:
       channel: master
       version_file: flutter_master.version
 
+  - name: Linux dart_unit_test_shard_1 master
+    bringup: true # New target
+    recipe: packages/packages
+    timeout: 60
+    properties:
+      target_file: dart_unit_tests.yaml
+      channel: master
+      version_file: flutter_master.version
+      cores: "32"
+      package_sharding: "--shardIndex 0 --shardCount 2"
+
+  - name: Linux dart_unit_test_shard_2 master
+    bringup: true # New target
+    recipe: packages/packages
+    timeout: 60
+    properties:
+      target_file: dart_unit_tests.yaml
+      channel: master
+      version_file: flutter_master.version
+      cores: "32"
+      package_sharding: "--shardIndex 1 --shardCount 2"
+
+  - name: Linux dart_unit_test_shard_1 stable
+    bringup: true # New target
+    recipe: packages/packages
+    timeout: 60
+    properties:
+      target_file: dart_unit_tests.yaml
+      channel: stable
+      version_file: flutter_stable.version
+      cores: "32"
+      package_sharding: "--shardIndex 0 --shardCount 2"
+
+  - name: Linux dart_unit_test_shard_2 stable
+    bringup: true # New target
+    recipe: packages/packages
+    timeout: 60
+    properties:
+      target_file: dart_unit_tests.yaml
+      channel: stable
+      version_file: flutter_stable.version
+      cores: "32"
+      package_sharding: "--shardIndex 1 --shardCount 2"
+
   - name: Linux analyze master
     recipe: packages/packages
     timeout: 30

.ci/scripts/dart_unit_tests_pathified.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+set -e
+
+# Pathify the dependencies on changed packages (excluding major version
+# changes, which won't affect clients).
+./script/tool_runner.sh make-deps-path-based --target-dependencies-with-non-breaking-updates
+# This uses --run-on-dirty-packages rather than --packages-for-branch
+# since only the packages changed by 'make-deps-path-based' need to be
+# re-checked.
+dart ./script/tool/bin/flutter_plugin_tools.dart dart-test --run-on-dirty-packages \
+    --log-timing --exclude=script/configs/dart_unit_tests_exceptions.yaml \
+    $PACKAGE_SHARDING
+# Restore the tree to a clean state, to avoid accidental issues if
+# other script steps are added to the enclosing task.
+git checkout .

.ci/targets/dart_unit_tests.yaml

@@ -0,0 +1,11 @@
+tasks:
+  - name: prepare tool
+    script: .ci/scripts/prepare_tool.sh
+  - name: Dart unit tests
+    script: script/tool_runner.sh
+    args: ["dart-test", "--exclude=script/configs/dart_unit_tests_exceptions.yaml"]
+    # Re-run tests with path-based dependencies to ensure that publishing
+    # the changes won't break tests of other packages in the respository
+    # that depend on it.
+  - name: Dart unit tests - pathified
+    script: .ci/scripts/dart_unit_tests_pathified.sh