This required switching from the Future-based bindings to the callback-based
bindings.

The new mojom.dart code makes mocking services a bit tricky. I've filed
https://github.com/domokit/mojo/issues/786 about improving that.

Also, add some missing docs to http.dart and widgets.dart.

Fixes #3730

These uses cases are now address by http.dart via http.readDataPipe.

We're getting back to the point where we have a bunch of foundation APIs
and it's getting confusing having them mixed with services/.

Previously we always used Latin-1.

- Create a FlutterErrorDetails struct-like class that describes an

  exception along with more details that aren't in the exception, like

  where it was caught and what was going on when it was caught.



- Provide a FlutterError static API for handling these objects:



  - FlutterError.onError which is called whenever Flutter catches an

    error.



  - FlutterError.reportError() which handles an error.



  - FlutterError.dumpErrorToConsole() which is the default behavior

    for onError.



- Removes all the existing exception handler callbacks.



- Replaces all the existing places that described exceptions using

  debugPrint with calls to FlutterError.reportError().



- Extend lockState() to also catch exceptions, so that we catch

  exceptions that happen during finalizers.



- Make the test framework catch errors and treat them as failures.



- Provide a mechanism to override this behavior in the test framework.



- Make the tests that used to depend on the exception handler

  callbacks use this new mechanism.



- Make pump() also support the phase argument.



- Improve some tests using these new features.



Fixes #2356, #2988, #2985, #2220.

We really shouldn't have two separate network library wrappers... I filed https://github.com/flutter/flutter/issues/2889 on that. But for now, this makes one of them have a generally better error handling story, and makes the other one only dump to the console in debug mode. It also makes both use debugPrint instead of print, and makes one use the 'mojom' prefix for mojom imports.

As usual, as I was doing this I ran into some stuff that seemed hard
to document as-is and so I changed it. In this case, in the "http"
library. The new code is more or less equivalent, I think, but the
resulting documentation makes it more obvious that it's wrong...

This is a variety of trivial fixes that I collected while going through the dartdocs.
This does contain some non-comment changes.