Move a bunch of src/ files into subdirectories.

lib/src/frontend/throws_matcher.dart

 // Copyright (c) 2014, 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.throws_matcher;
+library unittest.frontend.throws_matcher;
 
 import 'dart:async';
 
 import 'package:matcher/matcher.dart' hide fail, expect;
 
+import '../backend/invoker.dart';
+import '../utils.dart';
 import 'expect.dart';
-import 'invoker.dart';
-import 'utils.dart';
 
 /// This can be used to match two kinds of objects:
 ///
 /// * A [Function] that throws an exception when called. The function cannot
 ///   take any arguments. If you want to test that a function expecting
 ///   arguments throws, wrap it in another zero-argument function that calls
 ///   the one you want to test.
 ///
 /// * A [Future] that completes with an exception. Note that this creates an
 ///   asynchronous expectation. The call to `expect()` that includes this will
@@ skipping 79 matching lines @@
       mismatchDescription
           .add('threw ')
           .addDescriptionOf(matchState['exception']);
       if (verbose) {
         mismatchDescription.add(' at ').add(matchState['stack'].toString());
       }
       return mismatchDescription;
     }
   }
 }