revert: refactor($parse): only instantiate lex/parse once

This reverts commit 281feba.

Since Lexer and Parser objects are stateful it is not safe
to reuse them for parsing of multiple expressions.

After recent refactoring into prototypical style, the instantiation
of these objects is so cheap that it's not a huge win to use
singletons here.

src/ng/parse.js

@@ -1043,8 +1043,6 @@ function $ParseProvider() {
   var cache = {};
   this.$get = ['$filter', '$sniffer', function($filter, $sniffer) {
     return function(exp) {
-      var lexer = new Lexer($sniffer.csp);
-      var parser = new Parser(lexer, $filter, $sniffer.csp);
       var parsedExpression;
 
       switch (typeof exp) {
@@ -1053,6 +1051,8 @@ function $ParseProvider() {
             return cache[exp];
           }
 
+          var lexer = new Lexer($sniffer.csp);
+          var parser = new Parser(lexer, $filter, $sniffer.csp);
           parsedExpression = parser.parse(exp, false);
 
           if (exp !== 'hasOwnProperty') {