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

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index 8491a6894a63ae267a5dcdc418e3b79789e710bb..1abb8621a1bd8e41ee89bae504aa43693551f148 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -44,7 +44,8 @@ DEFINE_FLAG(bool, trace_parser, false, "Trace parser operations.");
 DEFINE_FLAG(bool, warn_mixin_typedef, true, "Warning on legacy mixin typedef.");
 DECLARE_FLAG(bool, throw_on_javascript_int_overflow);
 DECLARE_FLAG(bool, warn_on_javascript_compatibility);
-
+DEFINE_FLAG(bool, enable_mirrors, true,
+    "Disable to make importing dart:mirrors an error.");
 
 // Quick access to the current isolate and zone.
 #define I (isolate())
@@ -5783,6 +5784,11 @@ void Parser::ParseLibraryImportExport(intptr_t metadata_pos) {
     ReportError(import_pos, "private library is not accessible");
   }
 
+  if (!FLAG_enable_mirrors && Symbols::DartMirrors().Equals(canon_url)) {
+    ReportError(import_pos,
+                "import of dart:mirrors with --enable-mirrors=false");
+  }
+
   if (is_import) {
     if (prefix.IsNull() || (prefix.Length() == 0)) {
       ASSERT(!is_deferred_import);