refactor: emit warning for invalid & in XML

[vakrilov]

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What is the current behavior?

xml-namespace-loader throws error when encounters & in attribute value.

What is the new behavior?

xml-namespace-loader will only emit warning in this case. Having && in an attribute value might happen if you have a binding expression.

[nmandyam]

It is worrisome to see so many warnings about code that includes JS conditions ("a && b"). Is there a way we can hide these warnings during build?