[CP][stable channel] Improve messaging on invalid use of >>>

[johnniwinther]

commit(s) to merge:
01212e3
72de10d
1231e3d

merge instructions: https://dart-review.googlesource.com/c/sdk/+/213043

What is the issue: Using the >>> operator in a <2.14 library results in confusing parser errors because >>> is parsed as >> followed by a >, instead of mentioning that >>> needs the triple-shift language feature. Running dart on this program

// @dart=2.13
class Class {
  operator >>>(o) => null;
}
method(int i, int j) {
  i >>> j;
  i >>>= j;  
}

results in these confusing messages

test.dart:3:12: Error: A method declaration needs an explicit list of parameters.
Try adding a parameter list to the method declaration.
  operator >>>(o) => null;
           ^^
test.dart:3:14: Error: Expected '{' before this.
  operator >>>(o) => null;
             ^
test.dart:3:12: Error: Operator '>>' should have exactly one parameter.
  operator >>>(o) => null;
           ^^
test.dart:3:14: Error: Operator declarations must be preceded by the keyword 'operator'.
Try adding the keyword 'operator'.
  operator >>>(o) => null;
             ^
test.dart:6:7: Error: Expected an identifier, but got '>'.
Try inserting an identifier before '>'.
  i >>> j;
      ^
test.dart:7:7: Error: Expected an identifier, but got '>='.
Try inserting an identifier before '>='.
  i >>>= j;
      ^^

What is the fix: The fix is to special case >> immediately followed by > in the parser. This is done just after parsing operator > and just before parsing >> followed by > as a binary expression. At these points the normal message for an unavailable language feature is reported, instead the parser error it would normally have led to.

This means that after the fix, the output for the program above is:

test.dart:4:12: Error: This requires the 'triple-shift' language feature to be enabled.
Try updating your pubspec.yaml to set the minimum SDK constraint to 2.14 or higher, and running 'pub get'.
  operator >>>(o) => null;
           ^^^
test.dart:8:5: Error: This requires the 'triple-shift' language feature to be enabled.
Try updating your pubspec.yaml to set the minimum SDK constraint to 2.14 or higher, and running 'pub get'.
  i >>> j;
    ^^^
test.dart:9:5: Error: This requires the 'triple-shift' language feature to be enabled.
Try updating your pubspec.yaml to set the minimum SDK constraint to 2.14 or higher, and running 'pub get'.
  i >>>= j;
    ^^^^

Why cherrypick: Since the triple-shift feature is launched with the upcoming stable release, the messaging should clearly recognize failed attempts to use the features, and not indicate that the feature is unknown.

Risk: Low-medium risk. The parser change is added at points where parser errors are inevitable. The fix has been verified to work without cascading error messages.

Link to original issue(s): #46886

/cc @kevmoo @mit-mit @whesse @athomas @vsmenon @devoncarew @lrhn @jensjoha

[devoncarew]

Approved; cc @athomas.

[athomas]

Merged to stable in 3300f32 (2.14.2).