Removed IllegalAccessException and UnsupportedOperationException.

lib/compiler/implementation/lib/mirrors.dart

Index: lib/compiler/implementation/lib/mirrors.dart
diff --git a/lib/compiler/implementation/lib/mirrors.dart b/lib/compiler/implementation/lib/mirrors.dart
index f717d9b34a9df826a95c7a99c0ab702d17fb0493..243de2294cf21a01698cdb0c0b6f6986da389a9c 100644
--- a/lib/compiler/implementation/lib/mirrors.dart
+++ b/lib/compiler/implementation/lib/mirrors.dart
@@ -13,14 +13,14 @@
  */
 class _Mirrors {
   static MirrorSystem currentMirrorSystem() {
-    throw new UnsupportedOperationException("MirrorSystem not implemented");
+    throw new UnsupportedError("MirrorSystem not implemented");
   }
 
   static Future<MirrorSystem> mirrorSystemOf(SendPort port) {
-    throw new UnsupportedOperationException("MirrorSystem not implemented");
+    throw new UnsupportedError("MirrorSystem not implemented");
   }
 
   static InstanceMirror reflect(Object reflectee) {
-    throw new UnsupportedOperationException("MirrorSystem not implemented");
+    throw new UnsupportedError("MirrorSystem not implemented");
   }
 }