Add warning for undeclared property in template binding

[kevinpschaaf]

Description

In Polymer 1.x, any property bound in the template was automatically added to the list of attributes that are deserialized to properties.

In Polymer 2.x this changed, such that only properties declared in properties are added to observedAttributes and deserialized. This is because template parsing is lazy (deferred to first element instance) while observedAttributes must be present at registration time. That said, properties are used in templates still get setters and work as normal in terms of dynamic updates based on property changes; they just do not have corresponding observed attributes.

Since this is a silent difference between Polymer 1.x and 2.x, we propose adding a warning to the legacyOptimizations flag mode (to err on the conservative side, since users may have taken advantage of the terseness of not declaring properties and may not consider it a "bug" per se).

Versions

• Polymer: v2.x and 3.x