Implement Q1 2021 features. Add/update co19 tests for "Allow >>> as overridable operator"

[sgrekhov]

Add/update co19 tests for "Allow >>> as overridable operator" feature. Specification

Meta-issue dart-lang/sdk#44911

[alexmarkov]

Expression a >>> b in case of b < 0 results in a compile-time error if that evaluation happens at compile time - in constant context. Otherwise, the expression is evaluated at run time and throws a run-time error.

The test co19/src/Language/Expressions/Shift/integer_t04.dart (and co19_2/src/Language/Expressions/Shift/integer_t04.dart) incorrectly expects compile-time errors when expressions are not constants.

[alexmarkov]

The tests co19/src/LanguageFeatures/nnbd/triple-shift/local_variable_read_A02_t03.dart and co19/src/LanguageFeatures/nnbd/weak/triple-shift/local_variable_read_A02_t03.dart have the following expectation:

  Expect.throws(() {x >>>= 2;});

This expectation doesn't look correct. Why would it throw an exception? Maybe it should use y instead of x.

[iarkh]

Tests are already implemented in the Language and LanguageFeatures/Triple-Shift directories.