Add mapping for core-dropdown

[GabiAxel]

http://www.polymer-project.org/docs/elements/core-elements.html#core-dropdown
Added in Polymer 0.3.5

[DiegoBao]

After fighting a bit with the core-dropdown I manage to do it in this way:

First set the mappings for core-dropdown:

window.NG_POLYMER_ELEMENTS_EXTENDED_MAPPINGS = {
            coreDropdown: {
                ngModel: {
                    primitive: 'selected'
                }
            }
        };

Then:

<core-dropdown ng-model="mySelection">
    <core-item name="value1" label="Value 1"></core-item>
    <core-item name="value2" label="Value 2"></core-item>
</core-dropdown>

In this way the dropdown will set the variable mySelection with whatever is in the name attribute of core-item.
If name is not set, then it will set mySelection with the index of the element.

I tried to use valueattr of core-dropdown to set which attribute has the value to save but I couldn't make it work.

Hope this help

[GabiAxel]

Thanks, though it's already implemented in the head and will be included in the next release.