Poor recovery with comma in extends clause

[bwilkerson]

Enter the following code in a Dart file:

class A { }
class B { }
class Foo extends A, B {
  Foo() { }
}

This results in many errors being produced, but only one is useful: the extends clause takes a single type, not a list of types. We should have a single error with a better error message.

[sethladd]

Added Usability label.

[bwilkerson]

Removed Type-Defect, Usability labels.
Added Type-Enhancement, Analyzer-UX labels.

[bwilkerson]

Test for parser recovery added: https://dart-review.googlesource.com/c/sdk/+/37160

[srawlins]

Currently, the example generates 4 errors:

$ dart 22313.dart 
22313.dart:3:19: Error: A class declaration must have a body, even if it is empty.
Try adding an empty body.
class Foo extends A, B {
                  ^
22313.dart:3:20: Error: Expected a declaration, but got ','.
class Foo extends A, B {
                   ^
22313.dart:3:22: Error: A function declaration needs an explicit list of parameters.
Try adding a parameter list to the function declaration.
class Foo extends A, B {
                     ^
22313.dart:3:22: Error: 'B' is already declared in this scope.
class Foo extends A, B {
                     ^
22313.dart:2:7: Context: Previous declaration of 'B'.
class B { }
      ^