[Isolates] Allow running multiple isolates in shell and use this in tests.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 0e36cf36c8d5c8c237afaef2e0ee134ac7fc39f9..24aa060e0388496c9ee0264c4acf181bb234ccbe 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -633,7 +633,7 @@ class Isolate {
   char* ArchiveThread(char* to);
   char* RestoreThread(char* from);
 
-  static const char* kStackOverflowMessage;
+  static const char* const kStackOverflowMessage;
 
   // Accessors.
 #define GLOBAL_ACCESSOR(type, name, initialvalue)                              \
@@ -886,6 +886,9 @@ class Isolate {
 
   void Deinit();
 
+  static void SetIsolateThreadLocals(Isolate* isolate,
+                                     PerIsolateThreadData* data);
+
   enum State {
     UNINITIALIZED,    // Some components may not have been allocated.
     PREINITIALIZED,   // Components have been allocated but not initialized.