Improve parser recovery for trailing name in field declaration

[scheglov]

Code like int foo,; is currently parsed as having two variable declarations, which causes also FieldFragment with null name. But there is very little from the variable declaration, just a comma. And a better recovery would probably be just report that there is a not supported trailing comma, and don't create a new empty variable declaration.

  test_XXX() {
    var parseResult = parseStringWithErrors(r'''
class A {
  int foo,;
}
''');

    var node = parseResult.findNode.singleFieldDeclaration;
    assertParsedNodeText(node, r'''
FieldDeclaration
  fields: VariableDeclarationList
    type: NamedType
      name: int
    variables
      VariableDeclaration
        name: foo
      VariableDeclaration
        name: <empty> <synthetic>
  semicolon: ;
''');
  }