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

lib/src/runner/parse_metadata.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.
 
 library unittest.runner.parse_metadata;
 
 import 'dart:io';
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/src/generated/ast.dart';
@@ skipping 78 matching lines @@
       throw new SourceSpanFormatException(
           "TestOn takes a String.", _spanFor(args.first, path));
     }
 
     if (testOn != null) {
       throw new SourceSpanFormatException(
           "Only a single TestOn annotation may be used for a given test file.",
           _spanFor(annotation, path));
     }
 
-    testOn = args.first.stringValue;
+    testOn = args.first;
   }
 
-  return new Metadata.parse(testOn: testOn);
+  try {
+    return new Metadata.parse(
+        testOn: testOn == null ? null : testOn.stringValue);
+  } on SourceSpanFormatException catch (error) {
+    var file = new SourceFile(new File(path).readAsStringSync(),
+        url: p.toUri(path));
+    var span = contextualizeSpan(error.span, testOn, file);
+    if (span == null) rethrow;
+    throw new SourceSpanFormatException(error.message, span);
+  }
 }
 
 /// Creates a [SourceSpan] for [node].
 SourceSpan _spanFor(AstNode node, String path) =>
     // Load a SourceFile from scratch here since we're only ever going to emit
     // one error per file anyway.
     new SourceFile(new File(path).readAsStringSync(), url: p.toUri(path))
         .span(node.offset, node.end);