Do not check for an existing package map if a command is going to run "pub get" to update the package map (#3202)

Also remove a redundant check for the --pub option in the run command

This command explicitly populates the flutter tool's cache of binary artifacts.
Also, teach `flutter create` to update the cache in case its the first command
that a user runs.

This will help folks run pub get after they upgrade their flutter.git
revision.

* Added Scrollbar, removed ScrollbarPainter

* removed a dead import

* updated per review feedback

* Only call dispatch if the widgets State is still mounted

* some docs cleanup

* update styles

* remove unused file

Previously we'd snap to the size of the background widget before resizing to

zero. Now we force the background widget to match the size of the dimissable

widget at the time it was dismissed.



Fixes #3092

Closes https://github.com/flutter/flutter/issues/3149



Closes https://github.com/flutter/flutter/issues/3148

Previously we always used Latin-1.

* move the engine download code to dart

* fix populating the bin/cache/pkg dir

* document libs from sky_services

* scan directory of sky_services for libraries

* simplify error message when sky_services directory not found

* address review comments

Previously we lerped them backwards.



Fixes #2832

Add a command that can capture a Skia picture of a Flutter frame and send it to a file or skiaserve debugger (#3165)

* use --no-precompile

* use the flutter bin cache's sdk

Fixes https://github.com/flutter/flutter/issues/3094

Also, clean up a few interfaces that looked awkward when writing docs.

* add cache.dart to help manage the cache dir

* sp

They should automagically disable after the first tap.

Previously this was used by MixedViewport, but now we don't need it because

LazyBlockViewport has replaced MixedViewport.



I've also taken this opportunity to modernize RenderBlock.

Previously, we lost sync with the tab view contents when switching tabs. Now we
key the subtrees to make sure they keep their state across tab animations.

Fixes #3147

Overscroll indicator for MaterialList

See https://www.google.com/design/spec/style/writing.html#

Also, port the MixedViewport tests to LazyBlockViewport. Fix bugs found by the tests.

* add the dart vm version to flutter doctor

* drop the dart vm change

Fix some legit uses of GlobalKey to specify the type they want.



Fix some sketchy uses of GlobalKey in tests to fake it with "as

dynamic".



Remove some extraneous imports that made the build red.

Previously, the client of PopupMenuButton needed to build all the menu times

when building the PopupMenuButton. This can get expensive if, for example, each
item in a scrollable list has a popup menu associated with it.

Now the client passes a builder function to the PopupMenuButton that gets
invoked only when its time to show the menu items.

Fixes #2855

This patch adds dartdoc to LazyBlock. Also, this patch fixes the scrolling
physics of LazyBlock. Previously, we updated a running simulation only when the
change in scroll behavior changed the current scroll offset. Now we update
running simulations every time the behavior changes because the simulation
might depend on quantities other than the current scroll offset.

Add a `--no-codesign` option to `ios build` to disable code signing.

LazyBlock is going to replace ScrollableMixedWidgetList at some point.

Fix typo in Tooltip's shouldRelayout method.

LazyBlock is intended as a replacement for MixedViewport. Rather than

maintaining a table of all the observed child sizes (like

MixedViewport), LazyBlock works by dead reckoning the location of the

children based on the existing viewport. This approach makes it easier

to resize children because LazyBlock doesn't cache any additional

information that would need to be invalidated.



This patch contains a first draft of LazyBlock that works in a simple

usage scenario. Subsequent patches will replace

ScrollableMixedWidgetList with LazyBlock and port the existing

ScrollableMixedWidgetList tests over to LazyBlock.



Related to #3075