Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/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.
 
 library dart2js.compiler_base;
 
-import 'dart:async' show EventSink, Future;
+import 'dart:async' show Future;
 
 import '../compiler_new.dart' as api;
 import 'closure.dart' as closureMapping show ClosureTask;
 import 'common/names.dart' show Selectors;
 import 'common/names.dart' show Identifiers, Uris;
 import 'common/resolution.dart'
     show
         ParsingContext,
         Resolution,
         ResolutionWorkItem,
@@ skipping 14 matching lines @@
 import 'dump_info.dart' show DumpInfoTask;
 import 'elements/elements.dart';
 import 'elements/entities.dart';
 import 'elements/modelx.dart' show ErroneousElementX;
 import 'elements/resolution_types.dart'
     show
         ResolutionDartType,
         ResolutionDynamicType,
         ResolutionInterfaceType,
         Types;
-import 'enqueue.dart'
-    show DeferredAction, Enqueuer, EnqueueTask, ResolutionEnqueuer;
+import 'enqueue.dart' show Enqueuer, EnqueueTask, ResolutionEnqueuer;
 import 'environment.dart';
 import 'id_generator.dart';
 import 'io/source_information.dart' show SourceInformation;
 import 'js_backend/backend.dart' show JavaScriptBackend;
 import 'library_loader.dart'
     show
         ElementScanner,
         LibraryLoader,
         LibraryLoaderTask,
         LoadedLibraries,
@@ skipping 1725 matching lines @@
       compiler.readScript(uri, spannable);
 }
 
 /// [ScriptLoader] used to ensure that scripts are not loaded accidentally
 /// through the [LibraryLoader] when `CompilerOptions.compileOnly` is `true`.
 class _NoScriptLoader implements ScriptLoader {
   Compiler compiler;
   _NoScriptLoader(this.compiler);
 
   Future<Script> readScript(Uri uri, [Spannable spannable]) {
-    compiler.reporter
+    throw compiler.reporter
         .internalError(spannable, "Script loading of '$uri' is not enabled.");
   }
 }
 
 class _ElementScanner implements ElementScanner {
   ScannerTask scanner;
   _ElementScanner(this.scanner);
   void scanLibrary(LibraryElement library) => scanner.scanLibrary(library);
   void scanUnit(CompilationUnitElement unit) => scanner.scan(unit);
 }
@@ skipping 167 matching lines @@
     if (library != null && library.isSynthesized) {
       return null;
     }
     if (library == null && required) {
       throw new SpannableAssertionFailure(
           library, "The library '${uri}' was not found.");
     }
     return library;
   }
 }