Here's a template for how I plan on doing error-checking on inputs for

runtime/vm/dart_api_impl_test.cc

 // Copyright (c) 2011, 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 "vm/assert.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_api_state.h"
 #include "vm/unit_test.h"
@@ skipping 1773 matching lines @@
 static Dart_Handle library_handler(Dart_LibraryTag tag,
                                    Dart_Handle library,
                                    Dart_Handle url) {
   if (tag == kCanonicalizeUrl) {
     return url;
   }
   return Api::Success();
 }
 
 
+UNIT_TEST_CASE(LookupLibrary) {
+  const char* kScriptChars =
+      "#import('library1.dart');"
+      "main() {}";
+  const char* kLibrary1Chars =
+      "#library('library1.dart');"
+      "#import('library2.dart');";
+
+  Dart_CreateIsolate(NULL, NULL);
+  Dart_EnterScope();
+
+  // Create a test library and Load up a test script in it.
+  Dart_Handle url = Dart_NewString(TestCase::url());
+  Dart_Handle source = Dart_NewString(kScriptChars);
+  Dart_Handle result = Dart_LoadScript(url, source, library_handler);
+  EXPECT_VALID(result);
+
+  url = Dart_NewString("library1.dart");
+  source = Dart_NewString(kLibrary1Chars);
+  result = Dart_LoadLibrary(url, source);
+  EXPECT_VALID(result);
+
+  result = Dart_LookupLibrary(url);
+  EXPECT_VALID(result);
+
+  result = Dart_LookupLibrary(Dart_Null());
+  EXPECT(!Dart_IsValid(result));
+  EXPECT_STREQ("Dart_LookupLibrary expects argument 'url' to be non-null.",
+               Dart_GetError(result));
+
+  result = Dart_LookupLibrary(Dart_True());
+  EXPECT(!Dart_IsValid(result));
+  EXPECT_STREQ(
+      "Dart_LookupLibrary expects argument 'url' to be of type String.",
+      Dart_GetError(result));
+
+  result = Dart_LookupLibrary(Dart_Error("incoming error pass-thru"));
+  EXPECT(!Dart_IsValid(result));
+  EXPECT_STREQ("incoming error pass-thru", Dart_GetError(result));
+
+  url = Dart_NewString("noodles.dart");
+  result = Dart_LookupLibrary(url);
+  EXPECT(!Dart_IsValid(result));
+  EXPECT_STREQ("Dart_LookupLibrary: library 'noodles.dart' not found.",
+               Dart_GetError(result));
+
+  Dart_ExitScope();
+  Dart_ShutdownIsolate();
+}
+
+
 UNIT_TEST_CASE(ImportLibrary1) {
   const char* kScriptChars =
       "#import('library1.dart');"
       "#import('library2.dart');"
       "var foo;  // library2 defines foo, so should be error."
       "main() {}";
   const char* kLibrary1Chars =
       "#library('library1.dart');"
       "#import('library2.dart');"
       "var foo1;";
@@ skipping 243 matching lines @@
                                NULL);
     EXPECT_VALID(result);
 
     Dart_ExitScope();  // Exit the Dart API scope.
   }
   Dart_ShutdownIsolate();
 }
 #endif  // TARGET_ARCH_IA32.
 
 }  // namespace dart