Add native extensions for the Dart shell to mac and windows

runtime/bin/extensions.h

Index: runtime/bin/extensions.h
diff --git a/runtime/bin/extensions.h b/runtime/bin/extensions.h
index fda8f1e4d41754e1f9591496fdd7d55f262b33ac..830830d345990b7840b3d59554fff6dae3b433cd 100644
--- a/runtime/bin/extensions.h
+++ b/runtime/bin/extensions.h
@@ -10,12 +10,18 @@
 
 class Extensions {
  public:
-  // TODO(whesse): Make loading extensions lazy, so dynamic library is loaded
-  // only when first native function is called.
   // TODO(whesse): Make extension load from a relative path relative to
   // the library it is in.  Currently loads from current working directory.
   static Dart_Handle LoadExtension(const char* extension_url,
-                                   Dart_Handle library);
+                                   Dart_Handle parent_library);
+
+ private:
+  static char* MakeString(const char* format, const char* name);

[Mads Ager (google), 2012/03/08 12:31:29]

I did see the usages and I really don't like the generic name and the fact that
it takes a format that has be be very specific. String functions that take a
format usually uses varargs as well. I would rather have 

char* Concat(const char** strings);

which is general and should be easy to use in all the places where you use the
current MakeString. 

// Concatenates a NULL terminated array of strings.
char* Concat(const char** strings) {
  int size = 1;  // null termination.
  for (int i = 0; strings[i] != NULL; i++) {
    size += strlen(strings[i]);
  }
  char* result = reinterpret_cast<char*>(malloc(size));
  int index = 0;
  for (int i = 0; strings[i] != NULL; i++) {
    index += snprintf(result + index, size - index, "%s", strings[i]);
  }
  ASSERT(index == size - 1);
  ASSERT(result[size - 1] == '\0');
  return result;
}

Usages would be something like:

void MyMethod(const char* libname) {
  const char* strings[4] = { "lib", libname, ".so", NULL};
  UseTheConstructedName(Extensions::Concat(strings));
}

+
+  // Platform-specific implementations.
+  static void* LoadExtensionLibrary(const char* library_name);
+  static void* ResolveSymbol(void* lib_handle, const char* symbol);
+
  private:
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Extensions);