This issue was originally filed by @terrasea

What steps will reproduce the problem?

1. Create a Polymer template definition with an attribute then create the backing class without a field to match the attribute.
   <polymer-element name="failing-element" attributes="nonexistent">
   ...
   </polymer-element>

@CustomTag('failing-element')
class FailingElement extends PolymerElement {
  //don't create a definition for nonexistent
  ...
}
2. Try and run this in Dart Editor

What is the expected output? What do you see instead?
I expect a warning that I have not created a field in the backing class for this element. What I get instead is

Breaking on exception: Unhandled exception:

Exception: Unhandled exception:
Illegal argument(s): List must contain only ints, Strings, and Symbols

­0 PropertyPath.PropertyPath (package:observe/src/path_observer.dart:128:11)

­1 PolymerDeclaration.publishAttributes (package:polymer/src/declaration.dart:222:24)

­2 PolymerDeclaration.desugar (package:polymer/src/declaration.dart:107:22)

­3 PolymerDeclaration.register (package:polymer/src/declaration.dart:89:12)

­4 _hookJsPolymer.registerDart.<anonymous closure> (package:polymer/src/loader.dart:132:75)

­5 _RootZone.run (dart:async/zone.dart:1077)

­6 _hookJsPolymer.registerDart (package:polymer/src/loader.dart:131:22) (package:polymer/src/instance.dart:1

What version of the product are you using?

Dart Editor version 1.6.0.dev_08_00 (DEV)
Dart SDK version 1.6.0-dev.8.0

On what operating system?

Gentoo Linux

What browser (if applicable)?
Dartium

Please provide any additional information below.

I did a bit of tracking using, (horror), print statements. I found that it was going to packages/observe/src/path_observer.dart, which has been given a list of attributes, and on the failing list, the symbol checked was a null pointer. I guess it's something to do with the <polymer-element ...> attributes attribute. As when I removed that nonexistent attribute from the attributes attribute, my app started working again.