fix($parse): assign returns the new value

The `.assign` function returns the new value.
The version with csp enabled already has this behavior.

Closes #12675

Author: lgalfaso

src/ng/parse.js

@@ -774,6 +774,7 @@ ASTCompiler.prototype = {
       this.state.computing = 'assign';
       var result = this.nextId();
       this.recurse(assignable, result);
+      this.return_('v');
       extra = 'fn.assign=' + this.generateFunction('assign', 's,v,l');
     }
     var toWatch = getInputs(ast.body);

test/ng/parseSpec.js

@@ -2793,7 +2793,7 @@ describe('parser', function() {
           var fn = $parse('a');
           expect(fn.assign).toBeTruthy();
           var scope = {};
-          fn.assign(scope, 123);
+          expect(fn.assign(scope, 123)).toBe(123);
           expect(scope).toEqual({a:123});
         }));