Add a runtime flag to control availability of dart:mirrors.

runtime/vm/bootstrap.cc

Index: runtime/vm/bootstrap.cc
diff --git a/runtime/vm/bootstrap.cc b/runtime/vm/bootstrap.cc
index b844d0143fe8eabe712eb45536fb00ade464df10..f6c7f92fff4e7b8eedd6eb8a425fdf975b1c4806 100644
--- a/runtime/vm/bootstrap.cc
+++ b/runtime/vm/bootstrap.cc
@@ -16,6 +16,8 @@
 
 namespace dart {
 
+DECLARE_FLAG(bool, use_mirrors);
+
 #define INIT_LIBRARY(index, name, source, patch)                               \
   { index,                                                                     \
     "dart:"#name, source,                                                      \
@@ -273,6 +275,11 @@ RawError* Bootstrap::LoadandCompileScripts() {
        bootstrap_libraries[i].index_ != ObjectStore::kNone;
        ++i) {
     uri = Symbols::New(bootstrap_libraries[i].uri_);
+
+    if (!FLAG_use_mirrors && (uri.raw() == Symbols::DartMirrors().raw())) {
+      continue;
+    }
+
     lib = Library::LookupLibrary(uri);
     if (lib.IsNull()) {
       lib = Library::NewLibraryHelper(uri, false);
@@ -288,6 +295,11 @@ RawError* Bootstrap::LoadandCompileScripts() {
        bootstrap_libraries[i].index_ != ObjectStore::kNone;
        ++i) {
     uri = Symbols::New(bootstrap_libraries[i].uri_);
+
+    if (!FLAG_use_mirrors && (uri.raw() == Symbols::DartMirrors().raw())) {
+      continue;
+    }
+
     lib = Library::LookupLibrary(uri);
     ASSERT(!lib.IsNull());
     source = GetLibrarySource(lib, uri, false);