Fix Path.relativeTo in special case where base is a substring of this.

sdk/lib/_internal/dartdoc/lib/dartdoc.dart

Index: sdk/lib/_internal/dartdoc/lib/dartdoc.dart
diff --git a/sdk/lib/_internal/dartdoc/lib/dartdoc.dart b/sdk/lib/_internal/dartdoc/lib/dartdoc.dart
index 9b51ff461e9725ee59170e4b9aeb962b135caf5a..9470ecca0de80346ee047f7af6862307dfeed811 100644
--- a/sdk/lib/_internal/dartdoc/lib/dartdoc.dart
+++ b/sdk/lib/_internal/dartdoc/lib/dartdoc.dart
@@ -1836,20 +1836,7 @@ class Dartdoc {
       if (filename.endsWith('appcache.manifest')) {
         return;
       }
-      // TODO(johnniwinther): If [outputDir] has trailing slashes, [filename]
-      // contains double (back)slashes for files in the immediate [toCache]
-      // directory. These are not handled by [relativeTo] thus
-      // wrongfully producing the path `/foo.html` for a file `foo.html` in
-      // [toCache].
-      //
-      // This can be handled in two ways. 1) By ensuring that
-      // [Directory.fromPath] does not receive a path with a trailing slash, or
-      // better, by making [Directory.fromPath] handle such trailing slashes.
-      // 2) By ensuring that [filePath] does not have double slashes before
-      // calling [relativeTo], or better, by making [relativeTo] handle double
-      // slashes correctly.
-      Path filePath = new Path(filename).canonicalize();
-      Path relativeFilePath = filePath.relativeTo(outputDir);
+      Path relativeFilePath = new Path(filename).relativeTo(outputDir);
       write("$relativeFilePath\n");
     };
     toCacheLister.onDone = (done) => endFile();