Change how to opt-in to dart2js mirrors.

dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart

Index: dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart b/dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
index ae4646638ba549101bd5f9fbd90057511bd9cebd..68605212b9d64cee93d3686f63176a25ee61fe99 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
@@ -10,7 +10,6 @@ part 'constant_map.dart';
 part 'native_helper.dart';
 part 'regexp_helper.dart';
 part 'string_helper.dart';
-part 'mirror_opt_in_message.dart';
 
 // Performance critical helper methods.
 add(var a, var b) => (a is num && b is num)
@@ -399,8 +398,6 @@ class JSInvocationMirror implements InvocationMirror {
 class Primitives {
   static int hashCodeSeed = 0;
 
-  static bool mirrorsEnabled = false;
-
   static int objectHashCode(object) {
     int hash = JS('var', r'#.$identityHash', object);
     if (hash == null) {
@@ -417,11 +414,7 @@ class Primitives {
    * by defining a function in JavaScript called "dartPrint".
    */
   static void printString(String string) {
-    if ((MIRROR_OPT_IN_MESSAGE == string)) {
-      // Turn on mirrors. Also, make sure that this message isn't easy
-      // to suppress by not calling dartPrint.
-      mirrorsEnabled = true;
-    } else if (JS('bool', r'typeof dartPrint == "function"')) {
+    if (JS('bool', r'typeof dartPrint == "function"')) {
       // Support overriding print from JavaScript.
       JS('void', r'dartPrint(#)', string);
       return;