feat($parse): add support for ternary operators to parser

Add '?' token to lexer, add ternary rule to parser at
(hopefully) proper precedence and associativity (based
on https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/Operator_Precedence).
Since (exp1 && exp2 || exp3) is supported by the parser,
and (exp1 ? exp2 : exp3) works the same way, it seems
reasonable to add this minor form of control to templates
(see #719).

src/ng/parse.js

@@ -58,7 +58,7 @@ function lex(text, csp){
          (token=tokens[tokens.length-1])) {
         token.json = token.text.indexOf('.') == -1;
       }
-    } else if (is('(){}[].,;:')) {
+    } else if (is('(){}[].,;:?')) {
       tokens.push({
         index:index,
         text:ch,
@@ -359,6 +359,14 @@ function parser(text, json, $filter, csp){
     });
   }
 
+  function ternaryFn(left, middle, right){
+    return extend(function(self, locals){
+      return left(self, locals) ? middle(self, locals) : right(self, locals);
+    }, {
+      constant: left.constant && middle.constant && right.constant
+    });
+  }
+  
   function binaryFn(left, fn, right) {
     return extend(function(self, locals) {
       return fn(self, locals, left, right);
@@ -429,7 +437,7 @@ function parser(text, json, $filter, csp){
   }
 
   function _assignment() {
-    var left = logicalOR();
+    var left = ternary();
     var right;
     var token;
     if ((token = expect('='))) {
@@ -437,7 +445,7 @@ function parser(text, json, $filter, csp){
         throwError("implies assignment but [" +
           text.substring(0, token.index) + "] can not be assigned to", token);
       }
-      right = logicalOR();
+      right = ternary();
       return function(scope, locals){
         return left.assign(scope, right(scope, locals), locals);
       };
@@ -446,6 +454,24 @@ function parser(text, json, $filter, csp){
     }
   }
 
+  function ternary() {
+    var left = logicalOR();
+    var middle;
+    var token;
+    if((token = expect('?'))){
+      middle = ternary();
+      if((token = expect(':'))){
+        return ternaryFn(left, middle, ternary());
+      }
+      else {
+        throwError('expected :', token);
+      }
+    }
+    else {
+      return left;
+    }
+  }
+  
   function logicalOR() {
     var left = logicalAND();
     var token;