Add relative path to test when creating generated multitest directory

tools/testing/dart/multitest.dart

Index: tools/testing/dart/multitest.dart
diff --git a/tools/testing/dart/multitest.dart b/tools/testing/dart/multitest.dart
index eb85c1a0d86f58626bd7f4991f92f1ce699fb12a..b714e718e84412fefbadc5c2110b410677346906 100644
--- a/tools/testing/dart/multitest.dart
+++ b/tools/testing/dart/multitest.dart
@@ -245,7 +245,7 @@ Future doMultitest(
   ExtractTestsFromMultitest(filePath, tests, outcomes);
 
   Path sourceDir = filePath.directoryPath;
-  Path targetDir = CreateMultitestDirectory(outputDir, suiteDir);
+  Path targetDir = createMultitestDirectory(outputDir, suiteDir, sourceDir);
   assert(targetDir != null);
 
   // Copy all the relative imports of the multitest.
@@ -295,24 +295,19 @@ Future doMultitest(
   });
 }
 
-Path CreateMultitestDirectory(String outputDir, Path suiteDir) {
-  Directory generatedTestDir = new Directory('$outputDir/generated_tests');
-  if (!new Directory(outputDir).existsSync()) {
-    new Directory(outputDir).createSync();
-  }
-  if (!generatedTestDir.existsSync()) {
-    generatedTestDir.createSync();
-  }
+String suiteNameFromPath(Path suiteDir) {
   var split = suiteDir.segments();
+  // co19 test suite is at tests/co19/src.
   if (split.last == 'src') {
-    // TODO(sigmund): remove this once all tests are migrated to use
-    // TestSuite.forDirectory.
     split.removeLast();
   }
-  String path = '${generatedTestDir.path}/${split.last}';
-  Directory dir = new Directory(path);
-  if (!dir.existsSync()) {
-    dir.createSync();
-  }
-  return new Path(new File(path).absolute.path);
+  return split.last;
+}
+
+Path createMultitestDirectory(String outputDir, Path suiteDir, Path sourceDir) {
+  Path relative = sourceDir.relativeTo(suiteDir);
+  Path path = new Path(outputDir).append('generated_tests')
+      .append(suiteNameFromPath(suiteDir)).join(relative);
+  TestUtils.mkdirRecursive(TestUtils.currentWorkingDirectory, path);
+  return new Path(new File(path.toNativePath()).absolute.path);
 }