DOMAttributes, HTMLAttributes and SVGAttributes are all weak types.
In React.DOMElement, people always pass either HTMLAttributes or
SVGAttributes, so the type parameter is just constrained to
DOMAttributes. This is not as good as a union of (HTML | SVGAttrs) for 2
reasons:
1. Valid HTMLAttributes objects that don't include some DOMAttributes
properties fail TS 2.4's weak type check.
2. Invalid HTMLAttributes objects don't get type checking for
HTMLAttributes properties.
For example:
```ts
var e: React.DOMElement<{
style: {
checked: "no"
}
}, Element>;
```
But `checked` is actually a boolean. Previously, this was not an error.
These weak type errors were not caught in TS 2.4 RC. The final TS 2.4
will catch weak type errors with primitives, so this PR fixes those
now-caught errors.
