Add the content of the package.

lib/src/validator.dart

Index: lib/src/validator.dart
diff --git a/lib/src/validator.dart b/lib/src/validator.dart
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+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:source_span/source_span.dart';
+
+import 'ast.dart';
+import 'visitor.dart';
+
+typedef bool _IsDefined(String variable);
+
+/// An AST visitor that ensures that all variables are valid.
+class Validator extends RecursiveVisitor {
+  final _IsDefined _isDefined;
+
+  Validator(this._isDefined);
+
+  void visitVariable(VariableNode node) {
+    if (_isDefined(node.name)) return;
+    throw new SourceSpanFormatException("Undefined variable.", node.span);
+  }
+}