Support browser caching using hashes in serverMode (fixes #93, fixes #92)

lib/src/codegen/code_generator.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 dev_compiler.src.codegen.code_generator;
 
 import 'dart:io';
 
 import 'package:analyzer/src/generated/ast.dart' show CompilationUnit;
 import 'package:analyzer/src/generated/element.dart'
@@ skipping 111 matching lines @@
       return new Uri(path: suffix);
     }
     assert(index == 0);
     return new Uri(
         scheme: 'package', path: path.joinAll(parts.sublist(index + 1)));
   }
 
   CodeGenerator(String outDir, this.root, this.rules)
       : outDir = path.absolute(outDir);
 
-  void generateLibrary(Iterable<CompilationUnit> units, LibraryInfo info,
+  /// Return a hash, if any, that can be used for caching purposes. When two
+  /// invocations to this function return the same hash, the underlying
+  /// code-generator generated the same code.
+  String generateLibrary(Iterable<CompilationUnit> units, LibraryInfo info,
       CheckerReporter reporter);
 }