feat(directive injector): Assert that the injector has been initialized
refactor(Directive): remove all usages of deprecated Directive.*_VISIBILITY constants
refactor(Directive): Remove deprecated Directive.*_VISIBILITY constant

This changes is a BC break. The old constants should be replaced with
the new ones:

Before:

    Directive.LOCAL_VISIBILITY
    Directive.CHILDREN_VISIBILITY
    Directive.DIRECT_CHILDREN_VISIBILITY

After:

    Visibility.LOCAL
    Visibility.CHILDREN
    Visibility.DIRECT_CHILD
chore(changelog): update the changelog script
docs(CONTRIBUTING): fix minor grammar issues

Copyedit - fix minor grammar issues
fix(introspection): fix ngProbe with introspected bindings and findElements returning only elements

This change
1) organizes the introspection unit tests to more clearly separate them into
elementProbe tests and testability tests.

2) Modifies `interpolate` to return an `Interpolation` object which contains the
interpolated string and a list of binding expressions.

3) Adds an optional third parameter to findElements and findBindings which makes
the returned list include only elements or both elements and nodes (in practice,
these are text nodes). This defaults to only returning elements.
chore(di): update DI to 3.3.1
refactor(metadata extractor): remove a ref to Controller
doc(View): expand inline doc
doc(NgElement): add docs to NgElement
test(ng_model): reenable disabled test

Seems to be fine with dart v1.7

Closes #1236
fix(benchmark): update benchmark to work with new context.