Fail to load app if MXML Spark Label contains embedded newlines

Test case:

```

```

Causes this error on load:

```
Uncaught SyntaxError: '' string literal contains an unescaped line break TestSparkLabelBad.js:64:16
enter resource://devtools/server/actors/utils/event-loop.js:79
enter self-hosted:1161
_pushThreadPause resource://devtools/server/actors/thread.js:310
onExceptionUnwind resource://devtools/server/actors/thread.js:2013
onExceptionUnwind self-hosted:1161
TestSparkLabel_mx_managers_SystemManager .../bin/js-debug/TestSparkLabel_mx_managers_SystemManager.js:231
.../bin/js-debug/index.html:393
enter resource://devtools/server/actors/utils/event-loop.js:79
enter self-hosted:1161
```

Because this was generated during the compile:

```
Object.defineProperties(TestSparkLabelBad.prototype, /** @lends {TestSparkLabelBad.prototype} */ {
'MXMLDescriptor': {
/** @this {TestSparkLabelBad} */
get: function() {
if (this.mxmldd == undefined)
{
/** @type {Array} */
var arr = TestSparkLabelBad.superClass_.get__MXMLDescriptor.apply(this);
/** @type {Array} */
var data = [
spark.components.Label,
2,
'_id',
true,
'$ID_12_0',
'text',
true,
'Line1
Line2',
0,
0,
null
];
if (arr)
this.mxmldd = arr.concat(data);
else
this.mxmldd = data;
}
return this.mxmldd;
}
}
});
```

Reproduce the MXML snippet and inspect the generated TestSparkLabelBad.js MXMLDescriptor shown in the report. Trace how the embedded newline becomes the generated string literal; done means the generated JavaScript remains syntactically valid and the app loads with the label text preserved.

Written by the indexing model from the issue text.