refactor(*): faster check for a string starting with a substring

Thanks to @spamdaemon for the original PR to make this improvement.

In naive tests on Chrome I got the following results:

```
             Matches   Misses
indexOf      33ms      1494ms
lastIndexOf  11ms      11ms
```

Closes #3711

src/ng/location.js

@@ -48,17 +48,20 @@ function parseAppUrl(relativeUrl, locationObj) {
   }
 }
 
+function startsWith(haystack, needle) {
+  return haystack.lastIndexOf(needle, 0) === 0;
+}
 
 /**
  *
- * @param {string} begin
- * @param {string} whole
- * @returns {string} returns text from whole after begin or undefined if it does not begin with
- *                   expected string.
+ * @param {string} base
+ * @param {string} url
+ * @returns {string} returns text from `url` after `base` or `undefined` if it does not begin with
+ *                   the expected string.
  */
-function beginsWith(begin, whole) {
-  if (whole.indexOf(begin) === 0) {
-    return whole.substr(begin.length);
+function stripBaseUrl(base, url) {
+  if (startsWith(url, base)) {
+    return url.substr(base.length);
   }
 }
 
@@ -104,7 +107,7 @@ function LocationHtml5Url(appBase, appBaseNoFile, basePrefix) {
    * @private
    */
   this.$$parse = function(url) {
-    var pathUrl = beginsWith(appBaseNoFile, url);
+    var pathUrl = stripBaseUrl(appBaseNoFile, url);
     if (!isString(pathUrl)) {
       throw $locationMinErr('ipthprfx', 'Invalid url "{0}", missing path prefix "{1}".', url,
           appBaseNoFile);
@@ -141,14 +144,14 @@ function LocationHtml5Url(appBase, appBaseNoFile, basePrefix) {
     var appUrl, prevAppUrl;
     var rewrittenUrl;
 
-    if (isDefined(appUrl = beginsWith(appBase, url))) {
+    if (isDefined(appUrl = stripBaseUrl(appBase, url))) {
       prevAppUrl = appUrl;
-      if (isDefined(appUrl = beginsWith(basePrefix, appUrl))) {
-        rewrittenUrl = appBaseNoFile + (beginsWith('/', appUrl) || appUrl);
+      if (isDefined(appUrl = stripBaseUrl(basePrefix, appUrl))) {
+        rewrittenUrl = appBaseNoFile + (stripBaseUrl('/', appUrl) || appUrl);
       } else {
         rewrittenUrl = appBase + prevAppUrl;
       }
-    } else if (isDefined(appUrl = beginsWith(appBaseNoFile, url))) {
+    } else if (isDefined(appUrl = stripBaseUrl(appBaseNoFile, url))) {
       rewrittenUrl = appBaseNoFile + appUrl;
     } else if (appBaseNoFile == url + '/') {
       rewrittenUrl = appBaseNoFile;
@@ -182,14 +185,14 @@ function LocationHashbangUrl(appBase, appBaseNoFile, hashPrefix) {
    * @private
    */
   this.$$parse = function(url) {
-    var withoutBaseUrl = beginsWith(appBase, url) || beginsWith(appBaseNoFile, url);
+    var withoutBaseUrl = stripBaseUrl(appBase, url) || stripBaseUrl(appBaseNoFile, url);
     var withoutHashUrl;
 
     if (!isUndefined(withoutBaseUrl) && withoutBaseUrl.charAt(0) === '#') {
 
       // The rest of the url starts with a hash so we have
       // got either a hashbang path or a plain hash fragment
-      withoutHashUrl = beginsWith(hashPrefix, withoutBaseUrl);
+      withoutHashUrl = stripBaseUrl(hashPrefix, withoutBaseUrl);
       if (isUndefined(withoutHashUrl)) {
         // There was no hashbang prefix so we just have a hash fragment
         withoutHashUrl = withoutBaseUrl;
@@ -237,7 +240,7 @@ function LocationHashbangUrl(appBase, appBaseNoFile, hashPrefix) {
       var firstPathSegmentMatch;
 
       //Get the relative path from the input URL.
-      if (url.indexOf(base) === 0) {
+      if (startsWith(url, base)) {
         url = url.replace(base, '');
       }
 
@@ -300,7 +303,7 @@ function LocationHashbangInHtml5Url(appBase, appBaseNoFile, hashPrefix) {
 
     if (appBase == stripHash(url)) {
       rewrittenUrl = url;
-    } else if ((appUrl = beginsWith(appBaseNoFile, url))) {
+    } else if ((appUrl = stripBaseUrl(appBaseNoFile, url))) {
       rewrittenUrl = appBase + hashPrefix + appUrl;
     } else if (appBaseNoFile === url + '/') {
       rewrittenUrl = appBaseNoFile;
@@ -908,7 +911,7 @@ function $LocationProvider() {
     // update $location when $browser url changes
     $browser.onUrlChange(function(newUrl, newState) {
 
-      if (isUndefined(beginsWith(appBaseNoFile, newUrl))) {
+      if (isUndefined(stripBaseUrl(appBaseNoFile, newUrl))) {
         // If we are navigating outside of the app then force a reload
         $window.location.href = newUrl;
         return;

src/ngSanitize/sanitize.js

@@ -489,7 +489,7 @@ function stripCustomNsAttrs(node) {
     for (var i = 0, l = attrs.length; i < l; i++) {
       var attrNode = attrs[i];
       var attrName = attrNode.name.toLowerCase();
-      if (attrName === 'xmlns:ns1' || attrName.indexOf('ns1:') === 0) {
+      if (attrName === 'xmlns:ns1' || attrName.lastIndexOf('ns1:', 0) === 0) {
         node.removeAttributeNode(attrNode);
         i--;
         l--;