Make sure variable names used in status files are available in the environment

tools/testing/dart/status_expression.dart

 // Copyright (c) 2011, 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.
 
 library status_expression;
 
 /**
  * Parse and evaluate expressions in a .status file for Dart and V8.
  * There are set expressions and Boolean expressions in a .status file.
  * The grammar is:
@@ skipping 12 matching lines @@
  *
  *  Values and variableNames are non-empty strings of word characters, matching
  *  the RegExp \w+.
  *
  *  Expressions evaluate as expected, with values of variables found in
  *  an environment passed to the evaluator.  The SetExpression "value"
  *  evaluates to a singleton set containing that value. "A if B" evaluates
  *  to A if B is true, and to the empty set if B is false.
  */
 
+class ExprEvaluationException {
+  String error;
+
+  ExprEvaluationException(this.error);
+
+  toString() => error;
+}
 
 class Token {
   static const String LEFT_PAREN = "(";
   static const String RIGHT_PAREN = ")";
   static const String DOLLAR_SYMBOL = r"$";
   static const String UNION = ",";
   static const String EQUALS = "==";
   static const String NOT_EQUALS = "!=";
   static const String AND = "&&";
   static const String OR = "||";
@@ skipping 47 matching lines @@
   String toString() =>
       "(\$${left.name} ${negate ? '!=' : '=='} ${right.value})";
 }
 
 
 class TermVariable {
   String name;
 
   TermVariable(this.name);
 
-  String termValue(environment) => environment[name].toString();
+  String termValue(environment) {
+    var value = environment[name];
+    if (value == null) {
+      throw new ExprEvaluationException(
+          "Could not find '$name' in environment "
+          "while evaluating status file expression.");
+    }
+    return value.toString();
+  }
 }
 
 
 class TermConstant {
   String value;
 
   TermConstant(String this.value);
 
   String termValue(environment) => value;
 }
@@ skipping 195 matching lines @@
       }
       TermConstant right = new TermConstant(scanner.current);
       scanner.advance();
       return new Comparison(left, right, negate);
     } else {
       return new BooleanVariable(left);
     }
   }
 }