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

lib/src/codegen/code_generator.dart

Index: lib/src/codegen/code_generator.dart
diff --git a/lib/src/codegen/code_generator.dart b/lib/src/codegen/code_generator.dart
index cda50d156e6dfbad3081ba880ccf9dd3c30c20b4..c2920c49d109abc1ccbd74064bab522ebcb0b310 100644
--- a/lib/src/codegen/code_generator.dart
+++ b/lib/src/codegen/code_generator.dart
@@ -129,6 +129,9 @@ abstract class CodeGenerator {
   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);
 }