feat($interpolate): escaped interpolation expressions

This CL enables interpolation expressions to be escaped, by prefixing each character of their
start/end markers with a REVERSE SOLIDUS U+005C, and to render the escaped expression as a
regular interpolation expression.

Example:

`<span ng-init="foo='Hello'">{{foo}}, \\{\\{World!\\}\\}</span>` would be rendered as:
`<span ng-init="foo='Hello'">Hello, {{World!}}</span>`

This will also work with custom interpolation markers, for example:

     module.
       config(function($interpolateProvider) {
         $interpolateProvider.startSymbol('\\\\');
         $interpolateProvider.endSymbol('//');
       }).
       run(function($interpolate) {
         // Will alert with "hello\\bar//":
         alert($interpolate('\\\\foo//\\\\\\\\bar\\/\\/')({foo: "hello", bar: "world"}));
       });

This change effectively only changes the rendering of these escaped markers, because they are
not context-aware, and are incapable of preventing nested expressions within those escaped
markers from being evaluated.

Therefore, backends are encouraged to ensure that when escaping expressions for security
reasons, every single instance of a start or end marker have each of its characters prefixed
with a backslash (REVERSE SOLIDUS, U+005C)

Closes angular#5601

src/ng/interpolate.js

@@ -81,7 +81,15 @@ function $InterpolateProvider() {
 
   this.$get = ['$parse', '$exceptionHandler', '$sce', function($parse, $exceptionHandler, $sce) {
     var startSymbolLength = startSymbol.length,
-        endSymbolLength = endSymbol.length;
+        endSymbolLength = endSymbol.length,
+        escapedStartSymbol = startSymbol.replace(/./g, escape),
+        escapedEndSymbol = endSymbol.replace(/./g, escape),
+        escapedStartRegexp = new RegExp(escapedStartSymbol.replace(/./g, escape), 'g'),
+        escapedEndRegexp = new RegExp(escapedEndSymbol.replace(/./g, escape), 'g');
+
+    function escape(ch) {
+      return '\\' + ch;
+    }
 
     /**
      * @ngdoc service
@@ -180,6 +188,12 @@ function $InterpolateProvider() {
         separators.push('');
       }
 
+      forEach(separators, function(key, i) {
+        separators[i] = separators[i].
+          replace(escapedStartRegexp, startSymbol).
+          replace(escapedEndRegexp, endSymbol);
+      });
+
       // Concatenating expressions makes it hard to reason about whether some combination of
       // concatenated values are unsafe to use and could easily lead to XSS.  By requiring that a
       // single expression be used for iframe[src], object[src], etc., we ensure that the value

test/ng/interpolateSpec.js

@@ -61,6 +61,47 @@ describe('$interpolate', function() {
   }));
 
 
+  describe('interpolation escaping', function() {
+    var obj;
+
+    beforeEach(function() {
+      obj = {foo: 'Hello', bar: 'World'};
+    });
+
+    it('should support escaping interpolation signs', inject(function($interpolate) {
+      expect($interpolate('{{foo}} \\{\\{bar\\}\\}')(obj)).toBe('Hello {{bar}}');
+      expect($interpolate('\\{\\{foo\\}\\} {{bar}}')(obj)).toBe('{{foo}} World');
+    }));
+
+
+    it('should unescape multiple expressions', inject(function($interpolate) {
+      expect($interpolate('\\{\\{foo\\}\\}\\{\\{bar\\}\\} {{foo}}')(obj)).toBe('{{foo}}{{bar}} Hello');
+      expect($interpolate('{{foo}}\\{\\{foo\\}\\}\\{\\{bar\\}\\}')(obj)).toBe('Hello{{foo}}{{bar}}');
+      expect($interpolate('\\{\\{foo\\}\\}{{foo}}\\{\\{bar\\}\\}')(obj)).toBe('{{foo}}Hello{{bar}}');
+      expect($interpolate('{{foo}}\\{\\{foo\\}\\}{{bar}}\\{\\{bar\\}\\}{{foo}}')(obj)).toBe('Hello{{foo}}World{{bar}}Hello');
+    }));
+
+
+    it('should support customizing escape signs', function() {
+      module(function($interpolateProvider) {
+        $interpolateProvider.startSymbol('[[');
+        $interpolateProvider.endSymbol(']]');
+      });
+      inject(function($interpolate) {
+        expect($interpolate('[[foo]] \\[\\[bar\\]\\]')(obj)).toBe('Hello [[bar]]');
+      });
+    });
+
+
+    it('should unescape incomplete escaped expressions', inject(function($interpolate) {
+      expect($interpolate('\\{\\{foo{{foo}}')(obj)).toBe('{{fooHello');
+      expect($interpolate('\\}\\}foo{{foo}}')(obj)).toBe('}}fooHello');
+      expect($interpolate('foo{{foo}}\\{\\{')(obj)).toBe('fooHello{{');
+      expect($interpolate('foo{{foo}}\\}\\}')(obj)).toBe('fooHello}}');
+    }));
+  });
+
+
   describe('interpolating in a trusted context', function() {
     var sce;
     beforeEach(function() {