fix an exception in the outline view

Fix an exception in the outline view:
```
Exception: The null object does not have a getter 'bottom'.

NoSuchMethodError: method not found: 'bottom'
Receiver: null
Arguments: [] (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/outline.dart:25)
```

Also, add syntax highlighting for manifest.webapp files.

@ussuri
I don't think this implementation class (as most end node libraries in general) should assume that it knows the semantics of the higher-level class that uses it. `another == null` may mean something special to `Outline`, or it may be a programmatic error -- which is exactly what it is with this NPE. At most, these methods could do `assert(another != null)`, but that's superfluous with Dart.