Warn me if I use @observable without extends Object with ObservableMixin

[sethladd]

It would help certain users such as myself to be warned if they (me) use @­observable without also putting extends Object with ObservableMixin

[sigmundch]

=), this is basically what this TODO is all about:
https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/pkg/observe/lib/transform.dart#­148

I initially had more warnings, but it would have had false negatives (e.g. if you extend from Observable indirectly). So for now we only report a warning if there isn't a base class or mixin.

We could fix that if we did more work with the analyzer (resolving types and such).

[sethladd]

cc @sigmundch.
Added Library-PolymerLinter label.

[sigmundch]

Added this to the M8 milestone.

[sigmundch]

Removed this from the M8 milestone.
Added this to the Later milestone.

[sigmundch]

Removed this from the Later milestone.
Added this to the M9 milestone.

[clayberg]

Removed this from the M9 milestone.
Added this to the 1.1 milestone.

[sigmundch]

Removed this from the 1.1 milestone.
Added this to the 1.2 milestone.

[jmesserly]

if anything I think we're more likely to go the dirty-checking fallback route:
https://code.google.com/p/dart/issues/detail?id=16342

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Added NotPlanned label.