Avoid creating an iron-meta for every instance of iron-validatable

[sorvell]

An instance of IronMeta is created for every element that implements this behavior here: https://github.com/PolymerElements/iron-validatable-behavior/blob/master/iron-validatable-behavior.html#L84. To improve performance, the element should use one IronMeta or an IronMetaQuery and set type to validatorType before looking up the validator byKey. I'd also recommend seeing if this work can be done in an observer for the validator property rather than a getter.

[bicknellr]

:)