Add asserts that isolates have a root library set up before they become runnable.

runtime/lib/mirrors.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "lib/mirrors.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/bootstrap_natives.h"
 #include "vm/class_finalizer.h"
 #include "vm/compiler.h"
@@ skipping 337 matching lines @@
   args.SetAt(4, Bool::Get(cls.is_abstract()));
   args.SetAt(5, Bool::Get(cls.IsGeneric()));
   args.SetAt(6, Bool::Get(cls.is_mixin_app_alias()));
   args.SetAt(7, cls.NumTypeParameters() == 0 ? Bool::False() : is_declaration);
   args.SetAt(8, Bool::Get(cls.is_enum_class()));
   return CreateMirror(Symbols::_LocalClassMirror(), args);
 }
 
 
 static RawInstance* CreateLibraryMirror(const Library& lib) {
+  ASSERT(!lib.IsNull());
   const Array& args = Array::Handle(Array::New(3));
   args.SetAt(0, MirrorReference::Handle(MirrorReference::New(lib)));
   String& str = String::Handle();
   str = lib.name();
   args.SetAt(1, str);
   str = lib.url();
   if (str.Equals("dart:_builtin") || str.Equals("dart:_blink")) {
     // Censored library (grumble).
     return Instance::null();
   }
@@ skipping 1665 matching lines @@
 }
 
 DEFINE_NATIVE_ENTRY(TypeMirror_moreSpecificTest, 2) {
   GET_NON_NULL_NATIVE_ARGUMENT(AbstractType, a, arguments->NativeArgAt(0));
   GET_NON_NULL_NATIVE_ARGUMENT(AbstractType, b, arguments->NativeArgAt(1));
   return Bool::Get(a.IsMoreSpecificThan(b, NULL)).raw();
 }
 
 
 }  // namespace dart