Read builtin source files directly only under a flag which is off by default.

runtime/bin/builtin.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.
 
 #include <stdio.h>
 
 #include "include/dart_api.h"
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
@@ skipping 24 matching lines @@
     {"dart:web_audio", web_audio_source_paths_, NULL, NULL, true},
 #endif  // defined(DART_NO_SNAPSHOT)
 
     // End marker.
     {NULL, NULL, NULL, NULL, false}};
 
 Dart_Port Builtin::load_port_ = ILLEGAL_PORT;
 const int Builtin::num_libs_ =
     sizeof(Builtin::builtin_libraries_) / sizeof(Builtin::builtin_lib_props);
 
+static const bool use_builtin_source_paths = false;
+
 // Patch all the specified patch files in the array 'patch_files' into the
 // library specified in 'library'.
 static void LoadPatchFiles(Dart_Handle library,
                            const char* patch_uri,
                            const char** patch_files) {
   for (intptr_t j = 0; patch_files[j] != NULL; j += 3) {
-    Dart_Handle patch_src = DartUtils::ReadStringFromFile(patch_files[j + 1]);
+    Dart_Handle patch_src = Dart_Null();
+    if (use_builtin_source_paths) {
+      patch_src = DartUtils::ReadStringFromFile(patch_files[j + 1]);
+    }
     if (!Dart_IsString(patch_src)) {
-      // In case reading the file caused an error, use the sources directly.
+      // If use_builtin_source_paths is false or reading the file caused
+      // an error, use the sources linked in the binary.
       const char* source = patch_files[j + 2];
       patch_src = Dart_NewStringFromUTF8(
           reinterpret_cast<const uint8_t*>(source), strlen(source));
     }
 
     // Prepend the patch library URI to form a unique script URI for the patch.
     intptr_t len = snprintf(NULL, 0, "%s/%s", patch_uri, patch_files[j]);
     char* patch_filename = DartUtils::ScopedCString(len + 1);
     snprintf(patch_filename, len + 1, "%s/%s", patch_uri, patch_files[j]);
     Dart_Handle patch_file_uri = DartUtils::NewString(patch_filename);
@@ skipping 24 matching lines @@
 }
 
 
 Dart_Handle Builtin::GetSource(const char** source_paths, const char* uri) {
   if (source_paths == NULL) {
     return Dart_Null();  // No path mapping information exists for library.
   }
   for (intptr_t i = 0; source_paths[i] != NULL; i += 3) {
     if (!strcmp(uri, source_paths[i])) {
       const char* source_path = source_paths[i + 1];
-      Dart_Handle src = DartUtils::ReadStringFromFile(source_path);
+      Dart_Handle src = Dart_Null();
+      if (use_builtin_source_paths) {
+        src = DartUtils::ReadStringFromFile(source_path);
+      }
       if (!Dart_IsString(src)) {
-        // In case reading the file caused an error, use the sources directly.
+        // If use_builtin_source_paths is false or reading the file caused
+        // an error, use the sources linked in the binary.
         const char* source = source_paths[i + 2];
         src = Dart_NewStringFromUTF8(reinterpret_cast<const uint8_t*>(source),
                                      strlen(source));
       }
       return src;
     }
   }
   return Dart_Null();  // Uri does not exist in path mapping information.
 }
 
@@ skipping 53 matching lines @@
   Dart_Handle url = DartUtils::NewString(builtin_libraries_[id].url_);
   Dart_Handle library = Dart_LookupLibrary(url);
   if (Dart_IsError(library)) {
     library = LoadLibrary(url, id);
   }
   return library;
 }
 
 }  // namespace bin
 }  // namespace dart