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

tests/compiler/dart2js/memory_compiler.dart

Index: tests/compiler/dart2js/memory_compiler.dart
diff --git a/tests/compiler/dart2js/memory_compiler.dart b/tests/compiler/dart2js/memory_compiler.dart
index d3b183be08cda5c1f9a789a3732c74152387a927..400572524c5412ed1d4287bdac8747023d6b97b7 100644
--- a/tests/compiler/dart2js/memory_compiler.dart
+++ b/tests/compiler/dart2js/memory_compiler.dart
@@ -80,7 +80,11 @@ Future<CompilationResult> runCompiler(
     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,