path expressions containing 'in' no longer working, 'in' treated as keyword

**I'm submitting a bug report**

* **Library Version:**
2.1.7

**Please tell us about your environment:**
* **Operating System:**
Windows 10

* **Node Version:**
8.12.0

* **NPM Version:**
6.4.1

* **JSPM OR Webpack AND Version**
JSPM 0.17.0-beta.47

* **Browser:**
all

* **Language:**
ESNext

**Current behavior:**

```
Parser Error: Unexpected token in at column 0 in expression [in.name]
at ParserImplementation.err (aurelia-binding.js:2914)
at ParserImplementation.parseLeftHandSide (aurelia-binding.js:2651)
at ParserImplementation.parseBinary (aurelia-binding.js:2498)
at ParserImplementation.parseConditional (aurelia-binding.js:2487)
at ParserImplementation.parseExpression (aurelia-binding.js:2473)
at ParserImplementation.parseValueConverter (aurelia-binding.js:2455)
at ParserImplementation.parseBindingBehavior (aurelia-binding.js:2444)
at Parser.parse (aurelia-binding.js:2406)
at SyntaxInterpreter.bind (aurelia-templating-binding.js:533)
at SyntaxInterpreter.interpret (aurelia-templating-binding.js:505)
```

**Expected/desired behavior:**

It is shall be possible to use the 'in' keyword in path expressions, e.g.:
```value.bind="in.name"```

Prevents form upgrading form aurelia-binding version 1.6.0

Reproduce the failure with value.bind="in.name" using the reported Aurelia binding 2.1.7 environment, then trace the parser locations shown in aurelia-binding.js. Done means the path expression parses and binds successfully without treating the first path segment as a reserved keyword.

Written by the indexing model from the issue text.