Step one towards stable error messages with details:

pkg/polymer/lib/src/build/constants.dart

Index: pkg/polymer/lib/src/build/constants.dart
diff --git a/pkg/polymer/lib/src/build/constants.dart b/pkg/polymer/lib/src/build/constants.dart
new file mode 100644
index 0000000000000000000000000000000000000000..bc3753216e61f8749b5374e3a80338210e6d0d21
--- /dev/null
+++ b/pkg/polymer/lib/src/build/constants.dart
@@ -0,0 +1,20 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library polymer.src.build.constants;
+
+/// These names have meaning in SVG or MathML, so they aren't allowed as custom
+/// tags. See [isCustomTagName].
+const invalidTagNames = const {
+  'annotation-xml': '',
+  'color-profile': '',
+  'font-face': '',
+  'font-face-src': '',
+  'font-face-uri': '',
+  'font-face-format': '',
+  'font-face-name': '',
+  'missing-glyph': '',
+};
+
+const POLYMER_EXPERIMENTAL_HTML = 'packages/polymer/polymer_experimental.html';