go/parser: package name must not be the blank identifier

R=r, r2, rsc1
CC=golang-dev
https://golang.org/cl/4343045

src/pkg/go/parser/parser.go

@@ -2209,6 +2209,9 @@ func (p *parser) parseFile() *ast.File {
 	// Go spec: The package clause is not a declaration;
 	// the package name does not appear in any scope.
 	ident := p.parseIdent()
+	if ident.Name == "_" {
+		p.error(p.pos, "invalid package name _")
+	}
 	p.expectSemi()
 
 	var decls []ast.Decl