JSX: Convert encoded HTML entities in attributes into UTF-8

[huan086]

From #10492

Encountered another difference with babel regarding encoded HTML in attributes:

TypeScript Version: 2.0.0

Code

function MyComponent(props) {
    return (<div>{props.myAttr}</div>);
}

elsewhere:

<MyComponent myAttr="more&hellip;" />

Expected behavior:

Outputs the html

<div>more&hellip;</div>

Actual behavior:

Outputs the html

<div>more&amp;hellip;</div>

Make sure that &# and &#x works too!

Be careful of the following subtlety

<MyComponent myAttr="This is &amp;# encoded, like in HTML. \ has no effect" />
<MyComponent myAttr={"This is \\u encoded, like in JavaScript. & has no effect"} />

[ghost]

Fixed by #10747