1. Figure out the modified libraries from the specifeid kernel file during

runtime/bin/loader.cc

 // Copyright (c) 2016, 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 "bin/loader.h"
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
 #include "bin/dfe.h"
 #include "bin/extensions.h"
@@ skipping 655 matching lines @@
     if (Dart_IsError(library_url)) {
       return library_url;
     }
     return Dart_DefaultCanonicalizeUrl(library_url, url);
   }
   const char* url_string = NULL;
   Dart_Handle result = Dart_StringToCString(url, &url_string);
   if (Dart_IsError(result)) {
     return result;
   }
-  if (tag == Dart_kScriptTag) {
-    if (dfe.UseDartFrontend() || dfe.kernel_file_specified()) {
-      Dart_Isolate current = Dart_CurrentIsolate();
-      // Check if we are trying to reload a kernel file or if the '--dfe'
-      // option was specified and we need to compile sources using DFE.
-      if (!Dart_IsServiceIsolate(current) && !Dart_IsKernelIsolate(current)) {
-        // When using DFE the library tag handler should be called only when
-        // we are reloading scripts.
-        return dfe.ReloadScript(current, url_string);
-      }
-    }
-    // TODO(asiva) We need to ensure that the kernel and service isolates
-    // are always loaded from a kernel IR and do not use this path.
-  } else {
+  if (tag == Dart_kKernelTag) {
+    ASSERT(dfe.UseDartFrontend() || dfe.kernel_file_specified());
+    Dart_Isolate current = Dart_CurrentIsolate();
+    ASSERT(!Dart_IsServiceIsolate(current) && !Dart_IsKernelIsolate(current));
+    return dfe.ReadKernelBinary(current, url_string);
+  }
+  ASSERT(!dfe.UseDartFrontend() && !dfe.kernel_file_specified());
+  if (tag != Dart_kScriptTag) {
     // Special case for handling dart: imports and parts.
     // Grab the library's url.
     Dart_Handle library_url = Dart_LibraryUrl(library);
     if (Dart_IsError(library_url)) {
       return library_url;
     }
     const char* library_url_string = NULL;
     result = Dart_StringToCString(library_url, &library_url_string);
     if (Dart_IsError(result)) {
       return result;
     }
 
     bool is_dart_scheme_url = DartUtils::IsDartSchemeURL(url_string);
     bool is_dart_library = DartUtils::IsDartSchemeURL(library_url_string);
 
     if (is_dart_scheme_url || is_dart_library) {
       return DartColonLibraryTagHandler(tag, library, url, library_url_string,
                                         url_string);
     }
   }
-
   if (DartUtils::IsDartExtensionSchemeURL(url_string)) {
     // Handle early error cases for dart-ext: imports.
     if (tag != Dart_kImportTag) {
       return DartUtils::NewError("Dart extensions must use import: '%s'",
                                  url_string);
     }
     Dart_Handle library_url = Dart_LibraryUrl(library);
     if (Dart_IsError(library_url)) {
       return library_url;
     }
@@ skipping 213 matching lines @@
   MutexLocker ml(loader_infos_lock_);
   Loader* loader = LoaderForLocked(dest_port_id);
   if (loader == NULL) {
     return;
   }
   loader->QueueMessage(message);
 }
 
 }  // namespace bin
 }  // namespace dart