Document the triple-shift (>>>) operator

[kwalrath]

What information is missing?

2.14 will support the >>> operator. I need to update the language tour to reflect that. Resources:

• Feature specification
• Triple-shift operator language#120
• Implementation issue for unsigned/triple-shift >>> operator language#478
• Handle the unsigned shift operator in dart2js sdk#30890

The JS version is different because JS numbers are different:

• The feature spec says, "The JavaScript implementation of int's >>> operator must be decided and implemented by the JavaScript platforms. It likely works like >> except that it doesn't "sign-extend" the most significant bit."
• Lasse's comment says, "The int.>>> operator should probably do non-sign-extending shift of Int32 values, then convert to Uint32. That is where the Dart x >> y is implemented in JS as (x >> y) >>> 0, then x >>> y in Dart can be implemented as just x >>> y in JS."

How would you like us to fix this problem?

Language tour, where >> is listed/discussed.