Add code to contextualize a span relative to its original source file.

test/runner/runner_test.dart

 // Copyright (c) 2015, 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.
 
 @TestOn("vm")
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:unittest/src/util/exit_codes.dart' as exit_codes;
@@ skipping 110 matching lines @@
       var result = _runUnittest(["test.dart"]);
 
       expect(result.stderr, equals(
           'Failed to load "${p.relative(testPath, from: _sandbox)}":\n'
           "Error on line 1, column 8: TestOn takes one argument.\n"
           "@TestOn()\n"
           "       ^^\n"));
       expect(result.exitCode, equals(exit_codes.data));
     });
 
+    test("an annotation's contents are invalid", () {
+      var testPath = p.join(_sandbox, "test.dart");
+      new File(testPath).writeAsStringSync("@TestOn('zim')\nlibrary foo;");
+      var result = _runUnittest(["test.dart"]);
+
+      expect(result.stderr, equals(
+          'Failed to load "${p.relative(testPath, from: _sandbox)}":\n'
+          "Error on line 1, column 10: Undefined variable.\n"
+          "@TestOn('zim')\n"
+          "         ^^^\n"));
+      expect(result.exitCode, equals(exit_codes.data));
+    });
+
     test("a test file throws", () {
       var testPath = p.join(_sandbox, "test.dart");
       new File(testPath).writeAsStringSync("void main() => throw 'oh no';");
 
       var result = _runUnittest(["test.dart"]);
       expect(result.stderr, startsWith(
           'Failed to load "${p.relative(testPath, from: _sandbox)}": oh no\n'));
       expect(result.exitCode, equals(exit_codes.data));
     });
 
@@ skipping 118 matching lines @@
       expect(result.stdout, contains("\u001b[31m"));
     });
   });
 }
 
 ProcessResult _runUnittest(List<String> args) =>
     runUnittest(args, workingDirectory: _sandbox);
 
 ProcessResult _runDart(List<String> args) =>
     runDart(args, workingDirectory: _sandbox);