feat($compile): Allow late binding attributes

Sometimes is not desirable to use interpolation on attributes because
the user agent parses them before the interpolation takes place. I.e:

<svg>
  <circle cx="{{cx}}" cy="{{cy}}" r="{{r}}"></circle>
</svg>

The snippet throws three browser errors, one for each attribute.

For some attributes, AngularJS fixes that behaviour introducing special
directives like ng-href or ng-src.

This commit is a more general solution that allows prefixing any
attribute with "ng-attr-", "ng:attr:" or "ng_attr_"  so it will
be set only when the binding is done. The prefix is then removed.

I.e:
<svg>
  <circle ng_attr_cx="{{cx}}" ng-attr-cy="{{cy}}" ng:Attr:r="{{r}}"></circle>
</svg>

Closes angular#1050
Closes angular#1925

docs/content/guide/directive.ngdoc

@@ -488,6 +488,9 @@ link() or compile() functions - is a way of accessing:
     such as 'ng:bind', or 'x-ng-bind', the attributes object allows for normalized accessed to
     the attributes.
 
+  * *late binding attributes: If the attribute is prefixed with 'ng-attr-', 'ng:attr:' or 'ng_attr_'
+    the prefix will be removed once the attribute is evaluated, allowing late binding.
+
   * *directive inter-communication:* All directives share the same instance of the attributes
     object which allows the directives to use the attributes object as inter directive
     communication.

src/ng/compile.js

@@ -514,11 +514,16 @@ function $CompileProvider($provide) {
               directiveNormalize(nodeName_(node).toLowerCase()), 'E', maxPriority);
 
           // iterate over the attributes
-          for (var attr, name, nName, value, nAttrs = node.attributes,
+          for (var attr, name, nName, lName, value, nAttrs = node.attributes,
                    j = 0, jj = nAttrs && nAttrs.length; j < jj; j++) {
             attr = nAttrs[j];
             if (attr.specified) {
               name = attr.name;
+              // support late binding attributes
+              lName = directiveNormalize(name);
+              if (/ngAttr[A-Z]/.test(lName)) {
+                name = lName.substr(6).toLowerCase();
+              }
               nName = directiveNormalize(name.toLowerCase());
               attrsMap[nName] = name;
               attrs[nName] = value = trim((msie && name == 'href')

test/ng/compileSpec.js

@@ -2591,4 +2591,40 @@ describe('$compile', function() {
       });
     });
   });
+
+  describe('late binding attributes', function() {
+
+    it('should bind after digest but not before', inject(function($compile, $rootScope) {
+      $rootScope.name = "Misko";
+      element = $compile('<span ng-attr-test="{{name}}"></span>')($rootScope);
+      expect(element.attr('test')).toBeUndefined();
+      $rootScope.$digest();
+      expect(element.attr('test')).toBe('Misko');
+    }));
+
+    it('should work with different prefixes', inject(function($compile, $rootScope) {
+      $rootScope.name = "Misko";
+      element = $compile('<span ng:attr:test="{{name}}" ng-Attr-test2="{{name}}" ng_Attr_test3="{{name}}"></span>')($rootScope);
+      expect(element.attr('test')).toBeUndefined();
+      expect(element.attr('test2')).toBeUndefined();
+      expect(element.attr('test2')).toBeUndefined();
+      $rootScope.$digest();
+      expect(element.attr('test')).toBe('Misko');
+      expect(element.attr('test2')).toBe('Misko');
+      expect(element.attr('test3')).toBe('Misko');
+    }));
+
+    it('should work if they are prefixed with x- or data-', inject(function($compile, $rootScope) {
+      $rootScope.name = "Misko";
+      element = $compile('<span data-ng-attr-test2="{{name}}" x-ng-attr-test3="{{name}}" data-ng:attr-test4="{{name}}"></span>')($rootScope);
+      expect(element.attr('test2')).toBeUndefined();
+      expect(element.attr('test3')).toBeUndefined();
+      expect(element.attr('test4')).toBeUndefined();
+      $rootScope.$digest();
+      expect(element.attr('test2')).toBe('Misko');
+      expect(element.attr('test3')).toBe('Misko');
+      expect(element.attr('test4')).toBe('Misko');
+    }));
+  });
+
 });