Adds prefix slot that gets passed to paper-input.

[aarondrabeck]

Allows you to prefix stuff such as icons or emoji 😉 in the same way you can with a paper-input. Now your forms can have a consistent look with prefix icons.

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[aarondrabeck]

I signed it!

[googlebot]

CLAs look good, thanks!

[valdrinkoshi]

@aarondrabeck thanks for the contribution! Mind adding this to paper-dropdown-menu-light as well, and add 1 example in the demo/index.html?

[aarondrabeck]

@valdrinkoshi So it turns out putting this into paper-dropdown-menu-light is a bit of a rewrite. The issue is the label is currently using absolute position so when adding a variable width prefix the padding-left is indeterminate upfront. I would have to modify the component to conditionally use flow for the prefix, label, input and icon. This would significantly change the styles and animations that would closer resemble paper-input-container.

While I could do this, would this be something that should be done or should the light component simply not support prefix?

[valdrinkoshi]

Yeah, I realized that too :/ Another way to achieve the same is to use the mixin --paper-dropdown-menu-input if using paper-dropdown-menu-light, or --paper-dropdown-menu-input if using paper-dropdown-menu, to style the internal <input> to let space to the prefix element, e.g. http://jsbin.com/bidoyac/1/edit?html,output

Invoking @cdata @notwaldorf for opinions on this: would it be fine if paper-dropdown-menu supports setting a prefix element for the input, but paper-dropdown-menu-light doesn't?

[dnson]

@valdrinkoshi Do we have ETA when this PR can merge?