Prototype pollution in emit function

Summary

### Summary A prototype pollution in derby can crash the application, if the application author has atypical HTML templates that feed user input into an object key.

Attribute keys are almost always developer-controlled, not end-user-controlled, so this shouldn't be an issue in practice for most applications.

### Details ``` emit(context: Context, target: T) { const node = traverseAndCreate(context.controller, this.segments); node[this.lastSegment] = target; this.addListeners(target, node, this.lastSegment); } ``` The emit() function in src/templates/templates.ts is called without sanitizing the variable `this.lastSegment `. The variable `this.lastSegment ` can be set to `__proto__`, and this will pollute the prototype of Javascipt Object (`node['__proto__'] = target`).

### PoC To reproduce this vulnerability, you can adjust the test case `ignores DOM mutations in components\' create()` in `test/dom/ComponentHarness.mocha.js`.

``` it('ignores DOM mutations in components\' create()', function() { function Box() {} Box.view = { is: 'box', - source: '<index:><div class="box" as="boxElement"></div>' + source: '<index:><div class="box" as="__proto__"></div>' }; Box.prototype.create = function() { this.boxElement.className = 'box-changed-in-create'; }; var harness = runner.createHarness('<view is="box" />', Box); expect(harness).to.render('<div class="box"></div>'); }); ``` When `as` attribute is controlled by attackers, the variable in `this.lastSegment` will exactly take value` __proto__` and prototype pollution happens.

### Patch Add a check on `this.lastSegment` can prevent this attack. ``` emit(context: Context, target: T) { const node = traverseAndCreate(context.controller, this.segments); + if (this.lastSegment.includes('__proto__') || this.lastSegment.includes('prototype')) { + throw new Error('Unsafe code detected'); + } node[this.lastSegment] = target; this.addListeners(target, node, this.lastSegment); } ```