Property values that contain a : inside a mixin.

[blasten]

In a mixin, when you have a property which value contains a :, similar to:

:host {
    --paper-scroll-header-panel-full-header: {
        background-image: url(http://google.com/image.png);
    };
}

is converted to background-image: url(http:%20%20//google.com/image.png);

Reported in PolymerElements/paper-scroll-header-panel#4

[blasten]

this is fixed now in #1624