Use goog.isNull in if statements
Use goog.asserts.assertInstanceof to check the getSource return type

See 1499af397d9852eded193a8b06ea9d8bad71a49f
Merge pull request #1517 from fredj/vector-api-drawinteraction-cleanup

[vector-api] drawinteraction cleanup
Make ol.source.Source inherit from Observable
Add encoding param to goog.fs.FileReader.readAsText

This is a workaround for https://code.google.com/p/closure-library/issues/detail?id=524
Merge pull request #1516 from fredj/vector-api-source-observable

Make ol.source.Source inherit from Observable
Merge pull request #1518 from fredj/vector-api-1503

[vector-api] Add encoding param to goog.fs.FileReader.readAsText
snapshot