Site-www: Remove new & const (most) everywhere

[kwalrath]

• [x] Update language tour to avoid using new/const except where necessary. #507
• [x] Add optional new/const to /dart-2. #945
• [x] Update all tested code samples to NOT use new/const unless absolutely necessary. #966
• [x] Remove all uses of "new" and "const" from "Effective Dart". #844

[kwalrath]

We've updated all tested code samples, which means all of the most used/updated pages. @chalin, do you know whether everything else that might contain new and const is marked as obsolete?

[chalin]

do you know whether everything else that might contain new and const is marked as obsolete?

No, I don't know.

[chalin]

I'm checking now to ensure that pages that need to be are marked as (potentially) out-of-date (using obsolete: true).

[chalin]

A few extra pages needed to be marked as obsolete, see #973.

[chalin]

The analyzer is checking for unnecessary_const and unnecessary_new in all examples (#1026). No issues are reported, so I'd say that we're done!