Fix declaration of GenericTypeAliasElementForLink.
Fix declaration of GenericTypeAliasElementForLink.

This class needs to implement FunctionTypeAliasElementForLink so that
it can be stored in CompilationUnitElementForLink._functionTypeAliases
(and returned by CompilationUnitElementForLink.functionTypeAliases)
without incurring checked mode errors.

R=scheglov@google.com

Review-Url: https://codereview.chromium.org/2888653005 .

dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk