Fix a RegExp in analyzer_experimental/test/utils for Windows.

pkg/analyzer_experimental/test/utils.dart

Index: pkg/analyzer_experimental/test/utils.dart
diff --git a/pkg/analyzer_experimental/test/utils.dart b/pkg/analyzer_experimental/test/utils.dart
index 62f99095be6943ebc4efeb7e7ca9e433cf12327b..92bf53e5a98224e9e548a2205483a518f42f0e6f 100644
--- a/pkg/analyzer_experimental/test/utils.dart
+++ b/pkg/analyzer_experimental/test/utils.dart
@@ -25,7 +25,8 @@ String errorsForFile(String contents) {
       parseDartFile(path);
     } on AnalyzerErrorGroup catch (e) {
       return e.toString().replaceAllMapped(
-          new RegExp(r"^(Error on line \d+ of )([^:]+): .*$", multiLine: true),
+          new RegExp(r"^(Error on line \d+ of )((?:[A-Z]+:)?[^:]+): .*$",
+                     multiLine: true),
           (match) => match[1] + pathos.basename(match[2]) + ': ...');
     }
     return null;