Change test scripts to use Path library in most places, instead of strings.

tools/testing/dart/browser_test.dart

Index: tools/testing/dart/browser_test.dart
diff --git a/tools/testing/dart/browser_test.dart b/tools/testing/dart/browser_test.dart
index f7dccc21a67413fcdbd9fab73b482d8108aec445..7b52bc13fad639d3da8a14977785787ba0e13ad3 100644
--- a/tools/testing/dart/browser_test.dart
+++ b/tools/testing/dart/browser_test.dart
@@ -27,35 +27,18 @@ String GetHtmlContents(String title,
 </html>
 """;
 
-/**
- * Returns the native [path] converted for use in a URI.
- */
-nativePathToUri(String path) {
-  // This regexp matches Windows-like file names. Strictly speaking,
-  // this prevents us from having a file named a:something on Linux,
-  // but since this wrapping is a hack in the first place, it seems
-  // better to exercise this path on all architectures.
-  final re = const RegExp('^[a-z]:', ignoreCase: true);
-  if (re.hasMatch(path)) {
-    path = '/$path';
-  }
-  return path.replaceAll('\\', '/');
-}
-
-String WrapDartTestInLibrary(String test) =>
+String WrapDartTestInLibrary(Path test) =>
 """
 #library('libraryWrapper');
-#source('${nativePathToUri(test)}');
+#source('$test');
 """;
 
-String DartTestWrapper(String dartHome, String library) {
-  dartHome = nativePathToUri(dartHome);
-  library = nativePathToUri(library);
+String DartTestWrapper(Path dartHome, Path library) {
 return """
 #library('test');
 
-#import('${dartHome}/lib/unittest/unittest.dart', prefix: 'unittest');
-#import('${dartHome}/lib/unittest/html_config.dart', prefix: 'config');
+#import('${dartHome.append('lib/unittest/unittest.dart')}, prefix: 'unittest');
+#import('${dartHome.append('lib/unittest/html_config.dart')}, prefix: 'config');
 
 #import('${library}', prefix: "Test");