Create //sky/packages/workbench

The workbench package exists to pull in pub packages from both pub.dartlang.org
and from the source tree (using dependency overrides). The idea is that
workbench will reflect a typical Sky developer's environment so that we can use
the same tools as a typical sky developer by running them with the workbench as
the cwd.

engine/src/flutter/sky/packages/workbench/.gitignore

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+.pub
+packages
+pubspec.lock

engine/src/flutter/sky/packages/workbench/README.md

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+Workbench
+=========
+
+The workbench package is a synthetic package that lets us use `pub` to pull in
+some packages from `pub.dartlang.org` and some packages from within the source
+tree. The workbench package is intended to reflect a typical environment for
+developing a Sky application. 

engine/src/flutter/sky/packages/workbench/pubspec.yaml

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+name: workbench
+version: 0.0.1
+author: Chromium Authors <sky-dev@googlegroups.com>
+description: A workspace to host pub packages
+homepage: https://github.com/domokit/sky_engine/tree/master/sky/packages/workbench
+dependencies:
+  sky: any
+dev_dependencies:
+  sky_tools: ^0.0.3
+dependency_overrides:
+  material_design_icons:
+    path: ../material_design_icons
+  sky:
+    path: ../../sdk
+  sky_engine:
+    path: ../../../out/Debug/gen/dart-pkg/sky_engine
+  sky_services:
+    path: ../../../out/Debug/gen/dart-pkg/sky_services
+environment:
+  sdk: '>=1.8.0 <2.0.0'

engine/src/flutter/sky/packages/workbench/sky

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+../..