The peer pointer was not being setup at all, fixed that and added a test case

runtime/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 "bin/builtin.h"
 #include "include/dart_api.h"
 #include "include/dart_mirrors_api.h"
 #include "include/dart_native_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
@@ skipping 6952 matching lines @@
     str = Dart_MakeExternalString(canonical_str,
                                   ext_canonical_str,
                                   kLength,
                                   &canonical_str_peer,
                                   MakeExternalCback);
     EXPECT(Dart_IsString(str));
     EXPECT(!Dart_IsExternalString(canonical_str));
     EXPECT_EQ(canonical_str, str);
     EXPECT(Dart_IsString(canonical_str));
     EXPECT(!Dart_IsExternalString(canonical_str));
+    void* peer;
+    EXPECT_VALID(Dart_StringGetProperties(str, &size, &length, &peer));
+    EXPECT_EQ(1, size);
+    EXPECT_EQ(1, length);
+    EXPECT_EQ(reinterpret_cast<void*>(&canonical_str_peer), peer);
 
     // Test with a one byte ascii string.
     const char* ascii = "string";
     expected_length = strlen(ascii);
     Dart_Handle ascii_str = NewString(ascii);
     EXPECT_VALID(ascii_str);
     EXPECT(Dart_IsString(ascii_str));
     EXPECT(Dart_IsStringLatin1(ascii_str));
     EXPECT(!Dart_IsExternalString(ascii_str));
     EXPECT_VALID(Dart_StringLength(ascii_str, &length));
@@ skipping 298 matching lines @@
                                    NewString("main"),
                                    0,
                                    NULL);
   int64_t value = 0;
   result = Dart_IntegerToInt64(result, &value);
   EXPECT_VALID(result);
   EXPECT_EQ(8, value);
 }
 
 }  // namespace dart