[et] Lookup output filesystem path, not label

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Also breaks out functions for:
* `_gnDesc`: returns the JSON output of `gn desc buildDir target`
* `_gnOutputs`:  returns the output files listed by `gn outputs buildDir target`

Noticed while working on: flutter/flutter#147071

tools/engine_tool/lib/src/gn_utils.dart

@@ -77,30 +77,9 @@ final class BuildTarget {
 Future<Map<String, BuildTarget>> findTargets(
     Environment environment, Directory buildDir) async {
   final Map<String, BuildTarget> r = <String, BuildTarget>{};
-  final List<String> getBuildInfoCommandLine = <String>[
-    gnBinPath(environment),
-    'desc',
-    buildDir.path,
-    '*',
-    '--format=json',
-  ];
-
-  final ProcessRunnerResult result = await environment.processRunner.runProcess(
-      getBuildInfoCommandLine,
-      workingDirectory: environment.engine.srcDir,
-      failOk: true);
-
-  // Handle any process failures.
-  fatalIfFailed(environment, getBuildInfoCommandLine, result);
-
-  late final Map<String, Object?> jsonResult;
-  try {
-    jsonResult = jsonDecode(result.stdout) as Map<String, Object?>;
-  } catch (e) {
-    environment.logger.fatal(
-        'gn desc output could not be parsed:\nE=$e\nIN=${result.stdout}\n');
-  }
 
+  final Map<String, Object?> jsonResult =
+      await _runGnDesc(buildDir.path, '*', environment);
   for (final MapEntry<String, Object?> targetEntry in jsonResult.entries) {
     final String label = targetEntry.key;
     if (targetEntry.value == null) {
@@ -120,7 +99,7 @@ Future<Map<String, BuildTarget>> findTargets(
     }
     final bool testOnly = getBool(properties, 'testonly');
     final List<String> outputs =
-        getListOfString(properties, 'outputs') ?? <String>[];
+        await _runGnOutputs(buildDir.path, label, environment);
     File? executable;
     if (type == BuildTargetType.executable) {
       if (outputs.isEmpty) {
@@ -135,6 +114,55 @@ Future<Map<String, BuildTarget>> findTargets(
   return r;
 }
 
+/// Returns the JSON output of running `gn desc buildDir label`.
+Future<Map<String, Object?>> _runGnDesc(
+    String buildDir, String label, Environment environment) async {
+  final List<String> commandline = <String>[
+    gnBinPath(environment),
+    'desc',
+    buildDir,
+    label,
+    '--format=json',
+  ];
+
+  final ProcessRunnerResult result = await environment.processRunner.runProcess(
+      commandline,
+      workingDirectory: environment.engine.srcDir,
+      failOk: true);
+
+  // Handle any process failures.
+  fatalIfFailed(environment, commandline, result);
+
+  late final Map<String, Object?> jsonResult;
+  try {
+    jsonResult = jsonDecode(result.stdout) as Map<String, Object?>;
+  } catch (e) {
+    environment.logger.fatal(
+        'gn desc output could not be parsed:\nE=$e\nIN=${result.stdout}\n');
+  }
+  return jsonResult;
+}
+
+/// Returns the output paths returned by `gn outputs buildDir label`.
+Future<List<String>> _runGnOutputs(
+    String buildDir, String label, Environment environment) async {
+  final List<String> commandline = <String>[
+    gnBinPath(environment),
+    'outputs',
+    buildDir,
+    label,
+  ];
+  final ProcessRunnerResult result = await environment.processRunner.runProcess(
+      commandline,
+      workingDirectory: environment.engine.srcDir,
+      failOk: true);
+
+  // Handle any process failures.
+  fatalIfFailed(environment, commandline, result);
+
+  return result.stdout.split('\n');
+}
+
 /// Process selectors and filter allTargets for matches.
 ///
 /// We support:

tools/engine_tool/test/build_command_test.dart

@@ -361,10 +361,10 @@ void main() {
       ]);
       expect(result, equals(0));
       expect(testEnv.processHistory.length, greaterThanOrEqualTo(2));
-      expect(testEnv.processHistory[3].command[0], contains('ninja'));
-      expect(testEnv.processHistory[3].command[2], endsWith('/host_debug'));
+      expect(testEnv.processHistory[6].command[0], contains('ninja'));
+      expect(testEnv.processHistory[6].command[2], endsWith('/host_debug'));
       expect(
-        testEnv.processHistory[3].command[5],
+        testEnv.processHistory[6].command[5],
         equals('flutter/fml:fml_arc_unittests'),
       );
     } finally {
@@ -390,18 +390,18 @@ void main() {
       ]);
       expect(result, equals(0));
       expect(testEnv.processHistory.length, greaterThanOrEqualTo(2));
-      expect(testEnv.processHistory[3].command[0], contains('ninja'));
-      expect(testEnv.processHistory[3].command[2], endsWith('/host_debug'));
+      expect(testEnv.processHistory[6].command[0], contains('ninja'));
+      expect(testEnv.processHistory[6].command[2], endsWith('/host_debug'));
       expect(
-        testEnv.processHistory[3].command[5],
+        testEnv.processHistory[6].command[5],
         equals('flutter/display_list:display_list_unittests'),
       );
       expect(
-        testEnv.processHistory[3].command[6],
+        testEnv.processHistory[6].command[6],
         equals('flutter/flow:flow_unittests'),
       );
       expect(
-        testEnv.processHistory[3].command[7],
+        testEnv.processHistory[6].command[7],
         equals('flutter/fml:fml_arc_unittests'),
       );
     } finally {

tools/engine_tool/test/gn_utils_test.dart

@@ -39,6 +39,8 @@ void main() {
   final List<CannedProcess> cannedProcesses = <CannedProcess>[
     CannedProcess((List<String> command) => command.contains('desc'),
         stdout: fixtures.gnDescOutput()),
+    CannedProcess((List<String> command) => command.contains('outputs'),
+        stdout: 'display_list_unittests'),
   ];
 
   test('find test targets', () async {

tools/engine_tool/test/test_command_test.dart

@@ -42,6 +42,8 @@ void main() {
   final List<CannedProcess> cannedProcesses = <CannedProcess>[
     CannedProcess((List<String> command) => command.contains('desc'),
         stdout: fixtures.gnDescOutput()),
+    CannedProcess((List<String> command) => command.contains('outputs'),
+        stdout: 'display_list_unittests'),
   ];
 
   test('test command executes test', () async {
@@ -83,7 +85,7 @@ void main() {
         '//third_party/protobuf:protoc',
       ]);
       expect(result, equals(1));
-      expect(testEnvironment.processHistory.length, lessThan(3));
+      expect(testEnvironment.processHistory.length, lessThan(6));
       expect(testEnvironment.processHistory.where((ExecutedProcess process) {
         return process.command[0].contains('protoc');
       }), isEmpty);