Minor code reorg to ensure we don't include parts of the Library tag handler in the precompiled runtime builds.

R=rmacnak@google.com

Review-Url: https://codereview.chromium.org/2896523003 .
diff --git a/runtime/bin/loader.cc b/runtime/bin/loader.cc
index c939e70..cdf370b 100644
--- a/runtime/bin/loader.cc
+++ b/runtime/bin/loader.cc
@@ -617,7 +617,22 @@
                              payload_length);
 }
 
+#if defined(DART_PRECOMPILED_RUNTIME)
+Dart_Handle Loader::LibraryTagHandler(Dart_LibraryTag tag,
+                                      Dart_Handle library,
+                                      Dart_Handle url) {
+  return Dart_Null();
+}
 
+
+Dart_Handle Loader::DartColonLibraryTagHandler(Dart_LibraryTag tag,
+                                               Dart_Handle library,
+                                               Dart_Handle url,
+                                               const char* library_url_string,
+                                               const char* url_string) {
+  return Dart_Null();
+}
+#else
 Dart_Handle Loader::LibraryTagHandler(Dart_LibraryTag tag,
                                       Dart_Handle library,
                                       Dart_Handle url) {
@@ -803,6 +818,7 @@
   UNREACHABLE();
   return Dart_Null();
 }
+#endif  // !defined(DART_PRECOMPILED_RUNTIME)
 
 
 void Loader::InitOnce() {
diff --git a/runtime/bin/main.cc b/runtime/bin/main.cc
index b58f9b2..412a2fb2 100644
--- a/runtime/bin/main.cc
+++ b/runtime/bin/main.cc
@@ -114,7 +114,6 @@
 static char* app_script_uri = NULL;
 static const uint8_t* app_isolate_snapshot_data = NULL;
 static const uint8_t* app_isolate_snapshot_instructions = NULL;
-static AppSnapshot* app_snapshot = NULL;
 
 
 static Dart_Isolate main_isolate = NULL;
@@ -837,13 +836,16 @@
     }
   }
 
+  void* kernel_platform = NULL;
+  void* kernel_program = NULL;
+  AppSnapshot* app_snapshot = NULL;
+
 #if defined(DART_PRECOMPILED_RUNTIME)
   // AOT: All isolates start from the app snapshot.
   bool isolate_run_app_snapshot = true;
   const uint8_t* isolate_snapshot_data = app_isolate_snapshot_data;
   const uint8_t* isolate_snapshot_instructions =
       app_isolate_snapshot_instructions;
-  AppSnapshot* app_snapshot = NULL;
 #else
   // JIT: Main isolate starts from the app snapshot, if any. Other isolates
   // use the core libraries snapshot.
@@ -851,7 +853,6 @@
   const uint8_t* isolate_snapshot_data = core_isolate_snapshot_data;
   const uint8_t* isolate_snapshot_instructions =
       core_isolate_snapshot_instructions;
-  AppSnapshot* app_snapshot = NULL;
   if ((app_isolate_snapshot_data != NULL) &&
       (is_main_isolate || ((app_script_uri != NULL) &&
                            (strcmp(script_uri, app_script_uri) == 0)))) {
@@ -869,11 +870,6 @@
           &isolate_snapshot_data, &isolate_snapshot_instructions);
     }
   }
-#endif
-
-  void* kernel_platform = NULL;
-  void* kernel_program = NULL;
-#if !defined(DART_PRECOMPILED_RUNTIME)
   const bool is_service_isolate =
       strcmp(script_uri, DART_VM_SERVICE_ISOLATE_NAME) == 0;
   if (!is_kernel_isolate && !is_service_isolate) {
@@ -1658,7 +1654,7 @@
     Platform::Exit(kErrorExitCode);
   }
 
-  app_snapshot = Snapshot::TryReadAppSnapshot(script_name);
+  AppSnapshot* app_snapshot = Snapshot::TryReadAppSnapshot(script_name);
   if (app_snapshot != NULL) {
     vm_run_app_snapshot = true;
     app_snapshot->SetBuffers(&vm_snapshot_data, &vm_snapshot_instructions,