[dartdevc] Fixing windows-related source map pathing issues

Bug: #37364
Change-Id: If06a2a2737fbc55493ec3f0f607c3d75f7dc4546
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/107452
Reviewed-by: Jake Macdonald <jakemac@google.com>
Commit-Queue: Mark Zhou <markzipan@google.com>

Author: Markzipan

pkg/dev_compiler/lib/src/compiler/shared_command.dart

@@ -352,8 +352,8 @@ Map placeSourceMap(Map sourceMap, String sourceMapPath,
     {String multiRootOutputPath}) {
   var map = Map.from(sourceMap);
   // Convert to a local file path if it's not.
-  sourceMapPath = path.fromUri(sourcePathToUri(sourceMapPath));
-  var sourceMapDir = path.dirname(path.absolute(sourceMapPath));
+  sourceMapPath = sourcePathToUri(path.absolute(path.fromUri(sourceMapPath))).path;
+  var sourceMapDir = path.url.dirname(sourceMapPath);
   var list = (map['sources'] as List).toList();
 
   String makeRelative(String sourcePath) {

pkg/dev_compiler/lib/src/kernel/command.dart

@@ -136,8 +136,8 @@ Future<CompilerResult> _compile(List<String> args,
   var multiRootPaths = (argResults['multi-root'] as Iterable<String>)
       .map(Uri.base.resolve)
       .toList();
-  var multiRootOutputPath =
-      _longestPrefixingPath(path.absolute(output), multiRootPaths);
+  var multiRootOutputPath = _longestPrefixingPath(
+      sourcePathToUri(path.absolute(output)), multiRootPaths);
 
   var fileSystem = MultiRootFileSystem(
       multiRootScheme, multiRootPaths, fe.StandardFileSystem.instance);
@@ -518,7 +518,8 @@ String _findPackagesFilePath() {
 }
 
 /// Inputs must be absolute paths. Returns null if no prefixing path is found.
-String _longestPrefixingPath(String basePath, List<Uri> prefixingPaths) {
+String _longestPrefixingPath(Uri baseUri, List<Uri> prefixingPaths) {
+  var basePath = baseUri.path;
   return prefixingPaths.fold(null, (String previousValue, Uri element) {
     if (basePath.startsWith(element.path) &&
         (previousValue == null || previousValue.length < element.path.length)) {