Add a friendly error message for using a tab as indentation.

utils/tests/pub/yaml_test.dart

Index: utils/tests/pub/yaml_test.dart
diff --git a/utils/tests/pub/yaml_test.dart b/utils/tests/pub/yaml_test.dart
index 8616f488012945c79a87db26ab7c82bc0a049f26..c8d7facddcd48c8be286ca21491e94483bc87bb4 100644
--- a/utils/tests/pub/yaml_test.dart
+++ b/utils/tests/pub/yaml_test.dart
@@ -70,6 +70,21 @@ main() {
     });
   });
 
+  group('has a friendly error message for', () {
+    test('using a tab as indentation', () {
+      Expect.throws(() => loadYaml('foo:\n\tbar'),

[Bob Nystrom, 2012/08/30 16:43:51]

expect(() => loadYaml('foo:\n\tbar'),
    throwsA(contains('\\t is not allowed as indentation'));

[nweiz, 2012/08/30 19:19:02]

Done, although just "contains()" doesn't work here, since we're matching against
an exception rather than a string.

+          (e) => e.toString().contains('\\t is not allowed as indentation'));
+    });
+
+    test('using a tab not as indentation', () {
+      Expect.throws(() => loadYaml('''
+          "foo
+          \tbar"
+          error'''),
+        (e) => !e.toString().contains('\\t is not allowed as indentation'));

[Bob Nystrom, 2012/08/30 16:43:51]

Ditto.

We're trying to phase out all uses of Expect and eventually eliminate it from
corelib.

[nweiz, 2012/08/30 19:19:02]

Done.

+    });
+  });
+
   // The following tests are all taken directly from the YAML spec
   // (http://www.yaml.org/spec/1.2/spec.html). Most of them are code examples
   // that are directly included in the spec, but additional tests are derived