Let invalids eval. Works on #124.

lib/carto/tree/invalid.js

@@ -5,4 +5,16 @@ tree.Invalid = function Invalid(chunk, index, message) {
     this.type = 'syntax';
     this.message = message || "Invalid code: " + this.chunk;
 };
+
+tree.Invalid.prototype.eval = function(env) {
+    env.error({
+        chunk: this.chunk,
+        index: this.index,
+        type: 'syntax',
+        message: this.message || "Invalid code: " + this.chunk
+    });
+    return {
+        is: 'undefined'
+    };
+};
 })(require('../tree'));