Minor restructure some of the builtin library processing code

bin/gen_snapshot.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Generate a snapshot file after loading all the scripts specified on the
 // command line.
 
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
@@ skipping 159 matching lines @@
   if (Dart_IsError(result)) {
     return result;
   }
   // We only support canonicalization of "dart:".
   if (DartUtils::IsDartSchemeURL(url_string)) {
     if (tag == kCanonicalizeUrl) {
       return url;
     }
     ASSERT(tag == kImportTag);
     // Handle imports of other built-in libraries present in the SDK.
+    Builtin::BuiltinLibraryId id;
     if (DartUtils::IsDartCryptoLibURL(url_string)) {
-      return Builtin::LoadLibrary(Builtin::kCryptoLibrary);
+      id = Builtin::kCryptoLibrary;
     } else if (DartUtils::IsDartIOLibURL(url_string)) {
-      return Builtin::LoadLibrary(Builtin::kIOLibrary);
+      id = Builtin::kIOLibrary;
     } else if (DartUtils::IsDartJsonLibURL(url_string)) {
-      return Builtin::LoadLibrary(Builtin::kJsonLibrary);
+      id = Builtin::kJsonLibrary;
     } else if (DartUtils::IsDartUriLibURL(url_string)) {
-      return Builtin::LoadLibrary(Builtin::kUriLibrary);
+      id = Builtin::kUriLibrary;
     } else if (DartUtils::IsDartUtfLibURL(url_string)) {
-      return Builtin::LoadLibrary(Builtin::kUtfLibrary);
+      id = Builtin::kUtfLibrary;
+    } else {
+      return Dart_Error("Do not know how to load '%s'", url_string);
     }
+    return Builtin::LoadLibrary(id);
   }
   return Dart_Error("unexpected tag encountered %d", tag);
 }
 
 
 static Dart_Handle LoadGenericSnapshotCreationScript(
     Builtin::BuiltinLibraryId id) {
   Dart_Handle source = Builtin::Source(id);
   if (Dart_IsError(source)) {
     return source;  // source contains the error string.
@@ skipping 119 matching lines @@
     exit(255);
   }
   // Now write the snapshot out to specified file and exit.
   WriteSnapshotFile(buffer, size);
   Dart_ExitScope();
 
   // Shutdown the isolate.
   Dart_ShutdownIsolate();
   return 0;
 }