perf: use faster check for $$ prefix

Use two calls to charAt instead of substr to detect a $$prefix in the shallowCopy functions.
This makes shallowCopy 25-50% faster (depending on which browser is used).
http://jsperf.com/angular-shallow-copy

Closes angular#5457

src/Angular.js

@@ -769,7 +769,7 @@ function shallowCopy(src, dst) {
   for(var key in src) {
     // shallowCopy is only ever called by $compile nodeLinkFn, which has control over src
     // so we don't need to worry about using our custom hasOwnProperty here
-    if (src.hasOwnProperty(key) && key.substr(0, 2) !== '$$') {
+    if (src.hasOwnProperty(key) && key.charAt(0) !== '$' && key.charAt(1) !== '$') {
       dst[key] = src[key];
     }
   }

src/ngResource/resource.js

@@ -35,7 +35,7 @@ function shallowClearAndCopy(src, dst) {
   });
 
   for (var key in src) {
-    if (src.hasOwnProperty(key) && key.substr(0, 2) !== '$$') {
+    if (src.hasOwnProperty(key) && key.charAt(0) !== '$' && key.charAt(1) !== '$') {
       dst[key] = src[key];
     }
   }