Minor restructure some of the builtin library processing code

bin/builtin.h

Index: bin/builtin.h
===================================================================
--- bin/builtin.h	(revision 8133)
+++ bin/builtin.h	(working copy)
@@ -22,12 +22,17 @@
 class Builtin {
  public:
   enum BuiltinLibraryId {
-    kBuiltinLibrary,
+    // These libraries are included in the core snapshot.
+    kBuiltinLibrary = 0,
+    kJsonLibrary,
+    kUriLibrary,
+
+    // These libraries are not included in the core snapshot.
     kCryptoLibrary,
     kIOLibrary,
-    kJsonLibrary,
-    kUriLibrary,
     kUtfLibrary,
+
+    kInvalidLibrary,
   };
 
   static Dart_Handle Source(BuiltinLibraryId id);
@@ -48,6 +53,13 @@
   static const char uri_source_[];
   static const char utf_source_[];
 
+  typedef struct {
+    const char* url_;
+    const char* source_;
+    bool has_natives_;
+  } builtin_lib_props;
+  static builtin_lib_props builtin_libraries_[];
+
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Builtin);
 };