Fix some warnings.

lib/compiler/implementation/compiler.dart

Index: lib/compiler/implementation/compiler.dart
diff --git a/lib/compiler/implementation/compiler.dart b/lib/compiler/implementation/compiler.dart
index f2dbc25ef8caac3b115b7b26f9ee79801749a098..6e467421080a341e665af9b3c3ea9cea7ff7b978 100644
--- a/lib/compiler/implementation/compiler.dart
+++ b/lib/compiler/implementation/compiler.dart
@@ -57,7 +57,7 @@ class WorkItem {
   }
 }
 
-class Backend {
+abstract class Backend {
   final Compiler compiler;
   final ConstantSystem constantSystem;
 
@@ -78,6 +78,9 @@ class Backend {
   abstract void assembleProgram();
   abstract List<CompilerTask> get tasks;
 
+  // TODO(ahe,karlklose): rename this?
+  void dumpInferredTypes() {}
+
   ItemCompilationContext createItemCompilationContext() {
     return new ItemCompilationContext();
   }
@@ -85,7 +88,7 @@ class Backend {
   SourceString getCheckedModeHelper(DartType type) => null;
 }
 
-class Compiler implements DiagnosticListener {
+abstract class Compiler implements DiagnosticListener {
   final Map<String, LibraryElement> libraries;
   int nextFreeClassId = 0;
   World world;
@@ -592,18 +595,7 @@ class Compiler implements DiagnosticListener {
     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();
     }
   }