Fixes https://github.com/flutter/flutter/issues/6154

Fixes #6215

* Bump Dart SDK to `1.20.0-dev.10.0`.

`1.20.0-dev.10.0` corresponds to `1.20-RC0`.

* pubspec.lock cleanup

* check for pubspec.lock existence

* cleaned up conditional remove

* make flutter create less chatty

* output generated file count; mention lib/main.dart

This widget now lives in Mozart:

https://fuchsia.googlesource.com/mozart/+/master/lib/flutter/child_view.dart

This patch removes the Mozart dependencies from object.dart and
layer.dart, which will make it easier for us to move the Mozart
dependencies purely into the Fuchsia tree.

* Update ISSUE_TEMPLATE.md

Let's move the flutter doctor stuff last since it's usually not as critical as the logs.

* Update ISSUE_TEMPLATE.md

Fixes #988

The diagnostic port forward can be arbitrarily assigned and typically
is not the default

Fixes https://github.com/flutter/flutter/issues/5867

The current implementation matches the default behaviour of the material
scaffold.

Fixes https://github.com/flutter/flutter/issues/4478

fixes https://github.com/flutter/flutter/issues/6163

Refined query template with "new" keyword.

DragTarget would call setState after it was disposed in didLeave
and didDrop. Fixes #6128.

Turns out the lowest version allowed here isn't compatible with
flutter_tools' usage of this package.

Now dart:ui does the decoding of the pointer data itself, which means we don't
need to do it in the framework.