fix(mdAutocomplete): Compile autocomplete template against proper scope.

After the performance modifications to make autocomplete use the
virtual repeat directive, the autocomplete no longer compiled the
contents of the dropdown against the proper parent scope

This PR fixes it by manually compiling against the proper parent scope
and copying over the necessary `item` and `$index` properties.

Fixes angular#4390. Fixes angular#4495. Closes angular#4391.

src/components/autocomplete/autocomplete.spec.js

@@ -197,6 +197,48 @@ describe('<md-autocomplete>', function () {
 
       element.remove();
     }));
+
+    it('should compile the template against the parent scope', inject(function ($timeout) {
+      var scope    = createScope(null, { bang: 'boom' });
+      var template = '\
+          <md-autocomplete\
+              md-selected-item="selectedItem"\
+              md-search-text="searchText"\
+              md-items="item in match(searchText)"\
+              md-item-text="item.display"\
+              placeholder="placeholder">\
+            <md-item-template>\
+              <span class="find-parent-scope">{{bang}}</span>\
+              <span class="find-index">{{$index}}</span>\
+              <span class="find-item">{{item.display}}</span>\
+            </md-item-template>\
+          </md-autocomplete>';
+      var element  = compile(template, scope);
+      var ctrl     = element.controller('mdAutocomplete');
+      var ul       = element.find('ul');
+
+      expect(scope.bang).toBe('boom');
+
+      element.scope().searchText = 'fo';
+
+      // Run our initial flush
+      $timeout.flush();
+      waitForVirtualRepeat(element);
+
+      // Wait for the next tick when the values will be updated
+      $timeout.flush();
+
+      var li = ul.find('li')[0];
+
+      // Expect it to be compiled against the parent scope and have our variables copied
+      expect(li.querySelector('.find-parent-scope').innerHTML).toBe('boom');
+      expect(li.querySelector('.find-index').innerHTML).toBe('0');
+      expect(li.querySelector('.find-item').innerHTML).toBe('foo');
+
+      $timeout.flush();
+
+      element.remove();
+    }));
   });
 
   describe('xss prevention', function () {

src/components/autocomplete/js/autocompleteDirective.js

@@ -187,7 +187,7 @@ function MdAutocomplete () {
         var templateTag = element.find('md-item-template').detach(),
             html = templateTag.length ? templateTag.html() : element.html();
         if (!templateTag.length) element.empty();
-        return html;
+        return '<md-autocomplete-parent-scope md-autocomplete-replace>' + html + '</md-autocomplete-parent-scope>';
       }
 
       function getNoItemsTemplate() {

src/components/autocomplete/js/autocompleteParentScopeDirective.js

@@ -0,0 +1,34 @@
+angular
+  .module('material.components.autocomplete')
+  .directive('mdAutocompleteParentScope', MdAutocompleteItemScopeDirective);
+
+function MdAutocompleteItemScopeDirective($compile, $mdUtil) {
+  return {
+    restrict: 'AE',
+    link: postLink,
+    terminal: true
+  };
+
+  function postLink(scope, element, attr) {
+    var newScope = scope.$mdAutocompleteCtrl.parent.$new();
+    var relevantVariables = ['item', '$index'];
+
+    // Watch for changes to our scope's variables and copy them to the new scope
+    angular.forEach(relevantVariables, function(variable){
+      scope.$watch(variable, function(value) {
+        $mdUtil.nextTick(function() {
+          newScope[variable] = value;
+        });
+      });
+    });
+
+    // Recompile the contents with the new/modified scope
+    $compile(element.contents())(newScope);
+
+    // Replace it if required
+    if (attr.hasOwnProperty('mdAutocompleteReplace')) {
+      element.after(element.contents());
+      element.remove();
+    }
+  }
+}