Legacy warnings 3.x

[kevinpschaaf]

Adds warnings for a couple of subtle changes in behavior between 1.x and 2.x:

• Warn when re-declaring a computed property (this is no longer allowed in Polymer 2.x; see Overridden computed property functions do not correctly observe dependency properties #5468 (comment)). Fixes Overridden computed property functions do not correctly observe dependency properties #5468.
• Warn when using an undeclared property in a template binding (only when legacyOptimizations flag is enabled). Fixes Add warning for undeclared property in template binding #5473

Reference Issue

Fixes #5473 and #5468.

[sorvell]

Let's consider also warning if a user tries to change readOnly or reflectToAttribute or notify.

We should remove the doc line about "consider adding warnings" since we are doing that here.

[sorvell]

Suggested change

	console.warn(`Property '${prop}' used in template but not declared in 'properties'.`);
	console.warn(`Property '${prop}' used in template but not declared in 'properties'. Setting an attribute will not set the corresponding property unless the property is declared in 'properties'.`;

[kevinpschaaf]

Done

[sorvell]

LGTM, pending tests passing