Dart2JS fails to accept built-in identifier as variable name

[eernstg]

Dart2JS stops with the compile-time error 'Error: Expected identifier, but got ';'' in the co19 test

Language/12_Expressions/30_Identifier_Reference_A02_t01.

The text on ECMA-408 p86 implies that built-in identifiers should be allowed as names of variables, even though they are not allowed as type names. The error message could indicate that 'abstract' is parsed as a modifier and never considered as a potential variable name, but in any case it should not be an error.

[peter-ahe-google]

Duplicate of #6972.