Debugger: use a Map to cache mirrors.

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 5fee78310d4575e97d723900a44d590c8bf70603..1952e02ab5d1af2729b6087a7d6d4fcbc05953de 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1802,6 +1802,9 @@ Data* SetBuiltinTypedArray(Isolate* isolate, Handle<JSBuiltinsObject> builtins,
 
 
 void Genesis::InitializeBuiltinTypedArrays() {
+  // The serializer cannot serialize typed arrays. Reset those typed arrays
+  // for each new context.
+  DCHECK(!isolate()->serializer_enabled());
   Handle<JSBuiltinsObject> builtins(native_context()->builtins());
   {  // Initially seed the per-context random number generator using the
     // per-isolate random number generator.
@@ -3230,25 +3233,21 @@ Genesis::Genesis(Isolate* isolate,
   // Install experimental and extra natives. Do not include them into the
   // snapshot as we should be able to turn them off at runtime. Re-installing
   // them after they have already been deserialized would also fail.
-  if (context_type == FULL_CONTEXT) {
-    if (!isolate->serializer_enabled()) {
-      InitializeExperimentalGlobal();
+  if (!isolate->serializer_enabled() && context_type != THIN_CONTEXT) {
+    InitializeExperimentalGlobal();
+    InitializeBuiltinTypedArrays();
+    if (context_type == FULL_CONTEXT) {
       if (!InstallExperimentalNatives()) return;
       if (!InstallExtraNatives()) return;
       // By now the utils object is useless and can be removed.
       native_context()->set_natives_utils_object(
           isolate->heap()->undefined_value());
+      InitializeBuiltinTypedArrays();
+    } else {
+      DCHECK_EQ(DEBUG_CONTEXT, context_type);
+      if (!InstallDebuggerNatives()) return;
     }
-
-    // The serializer cannot serialize typed arrays. Reset those typed arrays
-    // for each new context.
-    InitializeBuiltinTypedArrays();
-  } else if (context_type == DEBUG_CONTEXT) {
-    DCHECK(!isolate->serializer_enabled());
-    InitializeExperimentalGlobal();
-    if (!InstallDebuggerNatives()) return;
   }
-
   result_ = native_context();
 }