Another try.

tools/testing/dart/test_suite.dart

Index: tools/testing/dart/test_suite.dart
diff --git a/tools/testing/dart/test_suite.dart b/tools/testing/dart/test_suite.dart
index 1970513e61ffce96f21b42adbc141f8313abcc1c..0f3f009c71fe297230e6d8de3a30daae3ebece1b 100644
--- a/tools/testing/dart/test_suite.dart
+++ b/tools/testing/dart/test_suite.dart
@@ -1517,7 +1517,7 @@ class TestUtils {
       // Ugly hack: if Windows absolute path starts with a drive,
       // rewrite [base] properly.
       if (segment.length == 2 && segment.endsWith(':')) {
-        base = new Path('${segment}\\');
+        base = new Path(segment);

[Bill Hesse, 2012/12/04 12:36:55]

I would use base = base.append(segment) here, like below.
Path('/').append('E:') is Path('/E:'), which is what you want as a path.  All
you want to do is skip the directory check and creation.

         continue;
       }
       base = base.append(segment);