chore(scope): explicitly declare variables in scope digest
feat(probe): Access injector, scope, directives from REPL

Add global methods to make it easier to debug Angular application from
the browser's REPL, unit or end-to-end tests.

Closes #305
chore(gitignore) add dart2js temp files to .gitignore
feat(ngForm): introduce basic support for the form directive
feat(probe): add ngQuery which can cross shadow root boundaries
fix(expression_extractor): support extracting expresions from attr mapping annotations

Closes #291