add `toBeAnInstanceOf` matcher

Since `isSubtypeOf` is supported in Dart2JS, the `toBeA` and `toThrowWith(type:)` matchers do not work when compiled to JS.  The `toBeAnInstanceOf` matcher is added to deal with it. It does a simpler check, which works in Dart2JS.
Merge pull request #14 from vsavkin/instance_of_matcher

add `toBeAnInstanceOf` matcher