Conflict with Object.prototype.watch in FireFox/Gecko

In Gecko engine `commandLine.options.watch` evaluates to a truthy value (a function). Adding an extra check to work around.

[Definition of CompilerOptions.watch in compiler/types](https://github.com/Microsoft/TypeScript/blob/master/src/compiler/types.ts#L1860)
``` typescript
    export interface CompilerOptions {
        // .  .  .
        watch?: boolean;
```

[Object.prototype.watch on MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/watch)
> Warning: Generally you should avoid using watch() and unwatch() when possible. These two methods are
> implemented only in Gecko, and they're intended primarily for debugging use. In addition, using watchpoints
> has a serious negative impact on performance, which is especially true when used on global objects, such
> as window. You can usually use setters and getters or proxies instead. See Browser compatibility for details.
> Also, do not confuse Object.watch with Object.observe.

Author: mihailik

src/compiler/tsc.ts

@@ -191,7 +191,7 @@ namespace ts {
             return sys.exit(ExitStatus.Success);
         }
 
-        if (commandLine.options.watch) {
+        if (commandLine.options.watch && commandLine.options.hasOwnProperty('watch')) { // FireFox has Object.prototype.watch
             if (!sys.watchFile) {
                 reportDiagnostic(createCompilerDiagnostic(Diagnostics.The_current_host_does_not_support_the_0_option, "--watch"));
                 return sys.exit(ExitStatus.DiagnosticsPresent_OutputsSkipped);