BREAKING CHANGE: `<portal>` has been renamed to `<teleport>`.
`target` prop is also renmaed to `to`, so the new usage will be:
```html
<Teleport to="#modal-layer" :disabled="isMobile">
<div class="modal">
hello
</div>
</Teleport>
```
The primary reason for the renaming is to avoid potential naming
conflict with [native portals](https://wicg.github.io/portals/).
Currently, when a component used is not properly registered, we have a warning and the vnode type is set to a Comment type in DEV mode. But in prod mode, we have no default value, making such an application broken and throw a strange error (`can not read _isSuspense of undefined`).
This commit avoids such an error in prod mode (as it is currently the case in Vue 2.x).
BREAKING CHANGE: adjust attr fallthrough behavior
Updated per pending RFC https://github.com/vuejs/rfcs/pull/137
- Implicit fallthrough now by default only applies for a whitelist
of attributes (class, style, event listeners, a11y attributes, and
data attributes).
- Fallthrough is now applied regardless of whether the component has
explicitly declared props. (close#749)
Previously codegen node for elements and components used raw expressions,
which leads to multiple permutations of AST shapes based on whether the
node is a block or has directives. The complexity is spread across the
entire compiler and occurs whenever a transform needs to deal with
element codegen nodes.
This refactor centralizes the handling of all possible permutations
into the codegen phase, so that all elements/components will have a
consistent node type throughout the transform phase.
The refactor is split into two commits (with test updates in a separate
one) so changes can be easier to inspect.
