Allow externalization of canonical strings also. Only restrict canonical strings

vm/dart_api_impl_test.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 "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
 #include "platform/utils.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart_api_impl.h"
@@ skipping 7388 matching lines @@
   const char* expected_str = "constant string";
   const intptr_t kExpectedLen = 15;
   int peer = 40;
   uint8_t ext_str[kExpectedLen];
   Dart_Handle str = Dart_MakeExternalString(result,
                                             ext_str,
                                             kExpectedLen,
                                             &peer,
                                             MakeExternalCback);
 
-  EXPECT(Dart_IsNull(str));
+  EXPECT(Dart_IsExternalString(str));
   for (intptr_t i = 0; i < kExpectedLen; i++) {
     EXPECT_EQ(expected_str[i], ext_str[i]);
   }
 }
 
 
 TEST_CASE(LazyLoadDeoptimizes) {
   const char* kLoadFirst =
       "start(a) {\n"
       "  var obj = (a == 1) ? createB() : new A();\n"
@@ skipping 27 matching lines @@
   Dart_LoadSource(TestCase::lib(), url, source);
 
   dart_args[0] = Dart_NewInteger(1);
   result = Dart_Invoke(lib1, NewString("start"), 1, dart_args);
   EXPECT_VALID(result);
 }
 
 #endif  // defined(TARGET_ARCH_IA32) || defined(TARGET_ARCH_X64).
 
 }  // namespace dart