Change executable name to "darfmt". Fix #163.

lib/src/source_visitor.dart

Index: lib/src/source_visitor.dart
diff --git a/lib/src/source_visitor.dart b/lib/src/source_visitor.dart
index 0fbb8da9bbe02dabc3c313e75ce3ab024850c6c7..69d1617a47952ac0eac09a803aa2c7c50155d7e8 100644
--- a/lib/src/source_visitor.dart
+++ b/lib/src/source_visitor.dart
@@ -1130,7 +1130,7 @@ class SourceVisitor implements AstVisitor {
     // Corner case: put a space between successive "-" operators so we don't
     // inadvertently turn them into a "--" decrement operator.
     if (node.operand is PrefixExpression &&
-        node.operand.operator.lexeme == "-") {
+        (node.operand as PrefixExpression).operator.lexeme == "-") {
       space();
     }