perf($compile): only iterate over elements with link functions

Closes #8741

Author: jbedard

src/ng/compile.js

@@ -987,9 +987,11 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
                   (nodeLinkFn.transcludeOnThisElement || !nodeLinkFn.templateOnThisElement)
                      && nodeLinkFn.transclude) : transcludeFn);
 
-        linkFns.push(nodeLinkFn, childLinkFn);
-        linkFnFound = linkFnFound || nodeLinkFn || childLinkFn;
-        nodeLinkFnFound = nodeLinkFnFound || nodeLinkFn;
+        if (nodeLinkFn || childLinkFn) {
+          linkFns.push(i, nodeLinkFn, childLinkFn);
+          linkFnFound = true;
+          nodeLinkFnFound = nodeLinkFnFound || nodeLinkFn;
+        }
 
         //use the previous context only for the first element in the virtual group
         previousCompileContext = null;
@@ -999,7 +1001,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
       return linkFnFound ? compositeLinkFn : null;
 
       function compositeLinkFn(scope, nodeList, $rootElement, parentBoundTranscludeFn) {
-        var nodeLinkFn, childLinkFn, node, childScope, i, ii, n, childBoundTranscludeFn;
+        var nodeLinkFn, childLinkFn, node, childScope, i, ii, idx, childBoundTranscludeFn;
         var stableNodeList;
 
 
@@ -1009,16 +1011,17 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
           var nodeListLength = nodeList.length;
           stableNodeList = new Array(nodeListLength);
 
-          for (i = 0; i < nodeListLength; i++) {
-            stableNodeList[i] = nodeList[i];
+          // create a sparse array by only copying the elements which have a linkFn
+          for (i = 0; i < linkFns.length; i+=3) {
+            idx = linkFns[i];
+            stableNodeList[idx] = nodeList[idx];
           }
         } else {
           stableNodeList = nodeList;
         }
 
-        // TODO(perf): when the DOM is sparsely annotated with directives, we spend a lot of time iterating over nulls here
-        for(i = 0, n = 0, ii = linkFns.length; i < ii; n++) {
-          node = stableNodeList[n];
+        for(i = 0, ii = linkFns.length; i < ii;) {
+          node = stableNodeList[linkFns[i++]];
           nodeLinkFn = linkFns[i++];
           childLinkFn = linkFns[i++];