Fix some warnings.

lib/compiler/implementation/compiler.dart

 // Copyright (c) 2012, 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.
 
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
 const bool REPORT_EXCESS_RESOLUTION = false;
@@ skipping 39 matching lines @@
   bool isAnalyzed() => resolutionTree !== null;
 
   void run(Compiler compiler, Enqueuer world) {
     CodeBuffer codeBuffer = world.universe.generatedCode[element];
     if (codeBuffer !== null) return;
     resolutionTree = compiler.analyze(this, world);
     compiler.codegen(this, world);
   }
 }
 
-class Backend {
+abstract class Backend {
   final Compiler compiler;
   final ConstantSystem constantSystem;
 
   Backend(this.compiler,
           [ConstantSystem constantSystem = DART_CONSTANT_SYSTEM])
       : this.constantSystem = constantSystem;
 
   void enqueueAllTopLevelFunctions(LibraryElement lib, Enqueuer world) {
     lib.forEachExport((Element e) {
       if (e.isFunction()) world.addToWorkList(e);
     });
   }
 
   abstract void enqueueHelpers(Enqueuer world);
   abstract void codegen(WorkItem work);
   abstract void processNativeClasses(Enqueuer world,
                                      Collection<LibraryElement> libraries);
   abstract void assembleProgram();
   abstract List<CompilerTask> get tasks;
 
+  // TODO(ahe,karlklose): rename this?
+  void dumpInferredTypes() {}
+
   ItemCompilationContext createItemCompilationContext() {
     return new ItemCompilationContext();
   }
 
   SourceString getCheckedModeHelper(DartType type) => null;
 }
 
-class Compiler implements DiagnosticListener {
+abstract class Compiler implements DiagnosticListener {
   final Map<String, LibraryElement> libraries;
   int nextFreeClassId = 0;
   World world;
   String assembledCode;
   Types types;
 
   final bool enableMinification;
   final bool enableTypeAssertions;
   final bool enableUserAssertions;
 
@@ skipping 486 matching lines @@
     backend.processNativeClasses(world, libraries.getValues());
     world.addToWorkList(main);
     progress.reset();
     world.forEach((WorkItem work) {
       withCurrentElement(work.element, () => work.run(this, world));
     });
     world.queueIsClosed = true;
     if (compilationFailed) return;
     assert(world.checkNoEnqueuedInvokedInstanceMethods());
     if (DUMP_INFERRED_TYPES && phase == PHASE_COMPILING) {
-      print("Inferred argument types:");
-      print("------------------------");
-      backend.argumentTypes.dump();
-      print("");
-      print("Inferred return types:");
-      print("----------------------");
-      backend.dumpReturnTypes();
-      print("");
-      print("Inferred field types:");
-      print("------------------------");
-      backend.fieldTypes.dump();
-      print("");
+      backend.dumpInferredTypes();
     }
   }
 
   /**
    * Perform various checks of the queues. This includes checking that
    * the queues are empty (nothing was added after we stopped
    * processing the queues). Also compute the number of methods that
    * were resolved, but not compiled (aka excess resolution).
    */
   checkQueues() {
@@ skipping 226 matching lines @@
   }
 
   Script readScript(Uri uri, [ScriptTag node]) {
     unimplemented('Compiler.readScript');
   }
 
   String get legDirectory {
     unimplemented('Compiler.legDirectory');
   }
 
+  // TODO(karlklose): split into findHelperFunction and findHelperClass and
+  // add a check that the element has the expected kind.
   Element findHelper(SourceString name)
       => jsHelperLibrary.findLocal(name);
   Element findInterceptor(SourceString name)
       => interceptorsLibrary.findLocal(name);
 
   bool get isMockCompilation => false;
 }
 
 class CompilerTask {
   final Compiler compiler;
@@ skipping 77 matching lines @@
  * information in the generated error message.
  */
 bool invariant(Spannable spannable, var condition, {String message: null}) {
   // TODO(johnniwinther): Use [spannable] and [message] to provide better
   // information on assertion errors.
   if (condition is Function){
     condition = condition();
   }
   return spannable != null && condition;
 }