fix(pub): update fails when project contains dependency from dart sdk

Packages such as analyzer depend on macros package, which depends on _macros which is included in the Dart SDK. When the project depends on such packages, Dependabot will no longer create any PRs because it cannot find _macros package.
This commit passes the DART_ROOT environment variable to dependency_services so that it can find the Dart SDK.

pub/lib/dependabot/pub/helpers.rb

@@ -229,6 +229,7 @@ def run_dependency_services(command, stdin_data: nil)
               "CI" => "true",
               "PUB_ENVIRONMENT" => "dependabot",
               "FLUTTER_ROOT" => "/tmp/flutter",
+              "DART_ROOT" => "/tmp/flutter/bin/cache/dart-sdk",
               "PUB_HOSTED_URL" => options[:pub_hosted_url],
               # This variable will make the solver run assuming that Dart SDK version.
               # TODO(sigurdm): Would be nice to have a better handle for fixing the dart sdk version.

pub/spec/dependabot/pub/update_checker_spec.rb

@@ -839,6 +839,28 @@
     end
   end
 
+  context "when given a project with dependencies from dart sdk" do
+    let(:project) { "can_update_with_dart_sdk_deps" }
+    let(:dependency_name) { "lints" }
+    let(:requirements_to_unlock) { :all }
+
+    it "can update lints" do
+      expect(can_update).to be_truthy
+      expect(updated_dependencies).to eq [
+        { "name" => "lints",
+          "package_manager" => "pub",
+          "previous_requirements" => [{
+            file: "pubspec.yaml", groups: ["dev"], requirement: "^3.0.0", source: nil
+          }],
+          "previous_version" => "3.0.0",
+          "requirements" => [{
+            file: "pubspec.yaml", groups: ["dev"], requirement: "^4.0.0", source: nil
+          }],
+          "version" => "4.0.0" }
+      ]
+    end
+  end
+
   context "when loading a YAML file with alias" do
     fixture = "spec/fixtures/projects/yaml_alias/"
     alias_info_file = "pubspec_alias_true.yaml"

pub/spec/fixtures/projects/can_update_with_dart_sdk_deps/pubspec.yaml

@@ -0,0 +1,8 @@
+name: dependabot_testcase
+environment:
+  sdk: ^3.5.0
+dependencies:
+  _macros:
+    sdk: dart
+dev_dependencies:
+  lints: ^3.0.0