tool_base 1.9.5+3

Base for developing command line apps. Take from Flutter Tools.

1.9.5+3

• Added ToolVersion to support tool versioning

1.9.5+2

• Synced with 1.9.1+hotfix.6 stable
• Added cache and net

1.9.5+1

• Fixed analysis warnings.

1.9.5

• Initial version.
  Partial contents of https://github.com/flutter/flutter/tree/v1.9.5/packages/flutter_tools/lib/src/base