Make source map location information more readable.

pkg/csslib/test/selector_test.dart

Index: pkg/csslib/test/selector_test.dart
diff --git a/pkg/csslib/test/selector_test.dart b/pkg/csslib/test/selector_test.dart
index f87136eead888a09ad6613adfb4e740f8b2a11c3..3f764e70a85a7e13dd643c3da87909f0b4e5bd07 100644
--- a/pkg/csslib/test/selector_test.dart
+++ b/pkg/csslib/test/selector_test.dart
@@ -54,10 +54,11 @@ void testSelectorFailures() {
   // Test for invalid class name (can't start with number).
   var selectorAst = selector('.foobar .1a-story .xyzzy', errors: errors);
   expect(errors.isEmpty, false);
-  expect(errors[0].toString(), r'''
-error :1:9: name must start with a alpha character, but found a number
-.foobar .1a-story .xyzzy
-        ^^''');
+  expect(errors[0].toString(),
+      'error on line 1, column 9: name must start with a alpha character, but '
+        'found a number\n'
+      '.foobar .1a-story .xyzzy\n'
+      '        ^^');
 }
 
 main() {