fix(virtualRepeat): Memory leak

Delete all created elements when the scope is destroyed.
Move cleanup code to cleanupBlocks().
Do not clean up used blocks.
Add unit tests.

Closes #8055. Closes #8056.

Author: neko1235

src/components/virtualRepeat/virtual-repeater.js

@@ -501,6 +501,8 @@ function VirtualRepeatController($scope, $element, $attrs, $browser, $document,
   this.blocks = {};
   /** @type {Array<!VirtualRepeatController.Block>} A pool of presently unused blocks. */
   this.pooledBlocks = [];
+
+  $scope.$on('$destroy', angular.bind(this, this.cleanupBlocks_));
 }
 
 
@@ -536,6 +538,14 @@ VirtualRepeatController.prototype.link_ =
 };
 
 
+/** @private Cleans up unused blocks. */
+VirtualRepeatController.prototype.cleanupBlocks_ = function() {
+  angular.forEach(this.pooledBlocks, function cleanupBlock(block) {
+    block.element.remove();
+  });
+};
+
+
 /** @private Attempts to set itemSize by measuring a repeated element in the dom */
 VirtualRepeatController.prototype.readItemSize_ = function() {
   if (this.itemSize) {

src/components/virtualRepeat/virtual-repeater.spec.js

@@ -658,6 +658,27 @@ describe('<md-virtual-repeat>', function() {
     }));
   });
 
+  describe('when container scope is destroyed', function() {
+
+    it('should clean up unused blocks', function() {
+      createRepeater();
+      var containerCtrl = component.controller('mdVirtualRepeatContainer');
+      scope.items = createItems(NUM_ITEMS);
+      scope.$apply();
+
+      scope.items = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'];
+      scope.$apply();
+
+      scope.$destroy();
+
+      var dataCount = 0;
+      angular.forEach(containerCtrl.repeater.pooledBlocks, function(block) {
+        dataCount += Object.keys(block.element.data()).length;
+      });
+      expect(dataCount).toBe(0);
+    });
+  });
+
   /**
    * Facade to access transform properly even when jQuery is used;
    * since jQuery's css function is obtaining the computed style (not wanted)