fix(mdContactChips): Compile contact chips against proper scope.

After the performance modifications to make autocomplete use the
virtual repeat directive, the contact chips no longer compiled the
contents of the dropdown against the proper scope, so it was loading
a set of blank elements in the autocomplete popup.

This is a hacky way to compile the contents against the proper scope
by creating a new child scope and merging the `item` property onto it
and synching any changes (which is very important since the virtual
repeat re-uses DOM elements).

Fixes #4390.

src/components/chips/js/contactChipsAutocompleteScopeDirective.js

@@ -0,0 +1,27 @@
+angular
+    .module('material.components.chips')
+    .directive('mdContactChipsAutocompleteScope', MdContactChipsAutocompleteScope);
+
+function MdContactChipsAutocompleteScope ($compile, $mdUtil) {
+  return {
+    restrict: 'A',
+    link:     postLink,
+    scope:    false
+  };
+
+  function postLink (scope, element, attr) {
+    // Grab the autocomplete controller's parent scope and create a new one
+    var newScope = scope.$mdAutocompleteCtrl.parent.$new();
+
+    // Watch for changes to our scope's item and copy it to the new scope
+    scope.$watch('item', function(item) {
+      $mdUtil.nextTick(function() {
+        newScope.item = item;
+      });
+    });
+
+    // TODO: transclude self might make it possible to do this without
+    // re-compiling, which is slow.
+    $compile(element.contents())(newScope);
+  }
+}

src/components/chips/js/contactChipsDirective.js

@@ -66,7 +66,7 @@ var MD_CONTACT_CHIPS_TEMPLATE = '\
               md-autoselect\
               placeholder="{{$mdContactChipsCtrl.contacts.length == 0 ?\
                   $mdContactChipsCtrl.placeholder : $mdContactChipsCtrl.secondaryPlaceholder}}">\
-            <div class="md-contact-suggestion">\
+            <div class="md-contact-suggestion" md-contact-chips-autocomplete-scope>\
               <img \
                   ng-src="{{item[$mdContactChipsCtrl.contactImage]}}"\
                   alt="{{item[$mdContactChipsCtrl.contactName]}}" />\