move dart_io_entries.txt to third_party/dart for AOT builds with a local engine (#12659)

packages/flutter_tools/lib/src/artifacts.dart

@@ -229,7 +229,7 @@ class LocalEngineArtifacts extends Artifacts {
   String getArtifactPath(Artifact artifact, [TargetPlatform platform, BuildMode mode]) {
     switch (artifact) {
       case Artifact.dartIoEntriesTxt:
-        return fs.path.join(_engineSrcPath, 'dart', 'runtime', 'bin', _artifactToFileName(artifact));
+        return fs.path.join(_engineSrcPath, 'third_party', 'dart', 'runtime', 'bin', _artifactToFileName(artifact));
       case Artifact.dartVmEntryPointsTxt:
         return fs.path.join(_engineSrcPath, 'flutter', 'runtime', _artifactToFileName(artifact));
       case Artifact.snapshotDart:

packages/flutter_tools/test/artifacts_test.dart

@@ -79,7 +79,7 @@ void main() {
     testUsingContext('getArtifactPath', () {
       expect(
           artifacts.getArtifactPath(Artifact.dartIoEntriesTxt, TargetPlatform.android_arm, BuildMode.debug),
-          fs.path.join(tempDir.path, 'dart', 'runtime', 'bin', 'dart_io_entries.txt')
+          fs.path.join(tempDir.path, 'third_party', 'dart', 'runtime', 'bin', 'dart_io_entries.txt')
       );
       expect(
           artifacts.getArtifactPath(Artifact.flutterFramework, TargetPlatform.ios, BuildMode.release),