Treat comments as not existing when checking for statements in the state machine rewriter
Removed support for custom keywords from the JS parser and instead added a "single line comments are statements" feature.
Merge branch 'roslyn' into source-maps
Fixed comment parse bug
Replaced JsLabelledStatement with JsLabel.

The JsLabel does not contain an embedded statement, but instead acts as an own statement. When formatting, an extra semicolon will be inserted to ensure valid syntax if a label is the last statement in a block.
Changed the state machine rewriter to no longer return if/else if chains when processing switch statements.

I can't see the rationale of the old behavior.
Full support for sequence point in the state machine rewriter
Test source locations in end-to-end state machine tests
Don't emit sequence points that are the same as the previously emitted one, and test that sequence points immediately followed by another sequence point are not emitted