Add "load from .dill" file capability and run a white-box test.

tests/compiler/dart2js/memory_compiler.dart

 // Copyright (c) 2016, 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.test.memory_compiler;
 
 import 'dart:async';
 
 import 'package:compiler/compiler.dart' show DiagnosticHandler;
 import 'package:compiler/compiler_new.dart'
@@ skipping 62 matching lines @@
     CompilerDiagnostics diagnosticHandler,
     CompilerOutput outputProvider,
     List<String> options: const <String>[],
     CompilerImpl cachedCompiler,
     bool showDiagnostics: true,
     Uri packageRoot,
     Uri packageConfig,
     PackagesDiscoveryProvider packagesDiscoveryProvider,
     void beforeRun(CompilerImpl compiler)}) async {
   if (entryPoint == null) {
-    entryPoint = Uri.parse('memory:main.dart');
+    if (options.contains('--read-dill')) {
+      entryPoint = Uri.parse('memory:main.dill');
+    } else {
+      entryPoint = Uri.parse('memory:main.dart');
+    }
   }
   CompilerImpl compiler = compilerFor(
       entryPoint: entryPoint,
       resolutionInputs: resolutionInputs,
       memorySourceFiles: memorySourceFiles,
       diagnosticHandler: diagnosticHandler,
       outputProvider: outputProvider,
       options: options,
       cachedCompiler: cachedCompiler,
       showDiagnostics: showDiagnostics,
@@ skipping 149 matching lines @@
       handler = (Uri uri, int begin, int end, String message, Diagnostic kind) {
         diagnosticHandler(uri, begin, end, message, kind);
         formattingHandler(uri, begin, end, message, kind);
       };
     }
   } else if (diagnosticHandler == null) {
     handler = (Uri uri, int begin, int end, String message, Diagnostic kind) {};
   }
   return handler;
 }