Revert r12379 (Add checks to external string API).

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 101361db0dab0248d7f7cc090e973cd8c49bf0db..65b544c7919eae9cb63119d7c0131b9be357ac73 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -537,13 +537,11 @@ Extension::Extension(const char* name,
     : name_(name),
       source_length_(source_length >= 0 ?
                      source_length :
-                     static_cast<int>(strlen(source))),
+                     (source ? static_cast<int>(strlen(source)) : 0)),
       source_(source, source_length_),
       dep_count_(dep_count),
       deps_(deps),
-      auto_enable_(false) {
-  CHECK(source);
-}
+      auto_enable_(false) { }
 
 
 v8::Handle<Primitive> Undefined() {