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

runtime/bin/builtin.cc

Index: runtime/bin/builtin.cc
diff --git a/runtime/bin/builtin.cc b/runtime/bin/builtin.cc
index cbc20063738280c799ba3b867b3f8c5f71196feb..73f54fd43c1e7d5cdaa568f4775aad312c16b3d0 100644
--- a/runtime/bin/builtin.cc
+++ b/runtime/bin/builtin.cc
@@ -42,15 +42,21 @@ 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));
@@ -95,9 +101,13 @@ Dart_Handle Builtin::GetSource(const char** source_paths, const char* uri) {
   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));