Adds missing context parameter

Eslint rule checks for missing preloads for stylesheets.

cc: @prateekbh 

- Upgrades cssnano to 1.2.0
- Only removes whitespaces from downloaded font declarations
- Disables Font optimization for AMP pages

We were accidentally allowing data requests to be rendered unconditionally. Instead, we should also check them against the staticPaths result and 404 when appropriate.

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Fixes #15383

Fixes #13953

This adds a `-d` or `--debug` flag which enables outputting additional build output information like the rewrites, redirects, and headers

Fixes #15281

Solves the warnings coming from mini-css-plugin in webpack 5.

Fixes #15947

The comment on the two different `--help` sections matched, though their logic if what was handled was slightly different.

This adjusts the comments to clarify that difference.

This is completely unrelated to function, but I noticed it while reviewing code and thought I'd PR the fix. 

This fixes page checking failing due to the trailing slash being present which causes pages to proxied by a rewrite when they shouldn't be. This also adds additional tests to ensure rewriting to an external resource is working correctly with `trailingSlash: true`

Fixes: https://github.com/vercel/next.js/issues/15700

This corrects the basePath being required check for filesystem routes to not consider the public folder catch-all route since it always matches even if the public file isn't present and instead moves the basePath check inside of the public-folder catch-all. Tests already exist that catch this by adding a public folder to the existing `basepath` test suite

Fixes: https://github.com/vercel/next.js/issues/16332
Closes: https://github.com/vercel/next.js/pull/16350

close #14744

<img width="851" alt="screen_shot" src="https://user-images.githubusercontent.com/39780486/86603491-06bec800-bfdf-11ea-9928-ee85cbad86a7.png">


I have some concerns.

- `import { isWriteable } from '../next/build/is-writeable'` **not** from `create-next-app ` package.
- The warning sentence is from npm. Not for Next.js. I'm not a native English speaker. I'd like to know the natural expression for this.

This PR reduces the total number of filesystem accesses we perform on CLI boot-up to make it happen faster.

This interpolates the dynamic values and rebuilds the request URL for fallback SSG pages since the proxy uses the output path for non-prerendered pages on Vercel which can cause inconsistent request URL/`asPath` values for SSG pages. This also adds tests to ensure the `asPath` is correctly updated

Fixes: https://github.com/vercel/next.js/issues/16269

Prior to this PR, `loadPage` would call `loadScript` which would then report if the script failed to load.

This was problematic because `loadScript` notified a failure to load via `pageRegisterEvents`, which would not set the `pageCache` value for future requests.
This means a one-off promise rejection would happen, [in lieu of being] typically consumed within the client-side router, causing a server-side reload.

However, when `loadPage` was used independently (i.e. to preload pages), this promise rejection would be ignored as a preload failure.
When the real routing request comes in, the `loadPage` function skips its attempt to load the `<script>` because it was already in the DOM, and the router would stop functioning.

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To fix this behavior, I've removed erroneous emits on `pageRegisterEvents` to only happen during the page registration lifecycle (its intended use).

The new behavior is that `loadScript` returns a `Promise` that `loadPage` can track, and if any of the page(s) scripts fail to load, we mark the entire page as errored in `pageCache`. This ensures future requests to `loadPage` will always immediately reject with a `PAGE_LOAD_ERROR`, which causes the server-side redirect at the appropriate point.

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Fixes #16333

Fixes a small error in the image preload post-processor. Adds a needed 'as' attribute to the link.

1. `as` should be set before `rel` in case of `preload` or `prefetch`
2. `href` should be after `onload` and `onerror`

This fixes an edge case where every dozen or so transitions you'll see a flash depending on what's happening on the main thread at the time.

I'm not sure it's possible to test for this case, so we'll just have to do more field testing with this.

This PR replaces `prop-types-exact` (only used in this location) with manual property checking.

Right now, malformed properties sent to `<Link>` are silently handled and only emit a warning in the console.
This leads to confusing/unexpected errors because we try to read a value that is undefined.

To fix this, we'll now throw a proper error when `<Link>` is misused. **This still isn't optimal, however, because we don't have a component stack trace we can give the user**.
We're not going to be able to give the user actionable instructions until React 16.14 at a minimum.

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Fixes #13951
Fixes #16107
Closes #13962

Fixes #16260

Don't use this yet as it's still being developed. This is a first iteration that enables the webpack cache. There's still more to do here, for example if css modules are used there's currently a bug where webpack does not save the cache for browser compilation (impacting build performance). @sokra is going to look into that issue.

Just correcting a simple typo `fash` to `fast`

Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>