Implement dynamicType and voidType as TypeMirrors not ClassMirrors. Issue 11743.

runtime/tests/vm/dart/isolate_mirror_local_test.dart

Index: runtime/tests/vm/dart/isolate_mirror_local_test.dart
===================================================================
--- runtime/tests/vm/dart/isolate_mirror_local_test.dart	(revision 24929)
+++ runtime/tests/vm/dart/isolate_mirror_local_test.dart	(working copy)
@@ -344,6 +344,10 @@
   testLibrariesMap(mirrors.libraries);
   Expect.equals(const Symbol('void'), mirrors.voidType.simpleName);
   Expect.equals(const Symbol('dynamic'), mirrors.dynamicType.simpleName);
+  Expect.isTrue(mirrors.voidType is TypeMirror);
+  Expect.isTrue(mirrors.dynamicType is TypeMirror);
+  Expect.isFalse(mirrors.voidType is ClassMirror);
+  Expect.isFalse(mirrors.dynamicType is ClassMirror);
   testDone('testMirrorSystem');
 }