Don't run main if library failed to import

Source/bindings/dart/DartApplicationLoader.cpp

Index: Source/bindings/dart/DartApplicationLoader.cpp
diff --git a/Source/bindings/dart/DartApplicationLoader.cpp b/Source/bindings/dart/DartApplicationLoader.cpp
index b95a191319c4a7f126210f81ff878f7ece6b4e37..0892f7e3bf0ae35391ee6d5521e08f820ec6be76 100644
--- a/Source/bindings/dart/DartApplicationLoader.cpp
+++ b/Source/bindings/dart/DartApplicationLoader.cpp
@@ -101,7 +101,11 @@ Dart_Handle DartApplicationLoader::libraryTagHandlerCallback(Dart_LibraryTag tag
 
 Dart_Handle DartApplicationLoader::libraryTagHandler(Dart_LibraryTag tag, Dart_Handle library, Dart_Handle urlHandle, String url)
 {
-    ASSERT(m_asyncLoader->contains(url));
+    if (!m_asyncLoader->contains(url)) {
+        scriptLoadError(url);
+        return Dart_NewBoolean(false);
+    }
+
     const String& source = m_asyncLoader->get(url);
 
     Dart_Handle import;
@@ -111,6 +115,7 @@ Dart_Handle DartApplicationLoader::libraryTagHandler(Dart_LibraryTag tag, Dart_H
         import = Dart_LoadSource(library, DartUtilities::stringToDartString(url), DartUtilities::convertSourceString(source));
     } else {
         ASSERT_NOT_REACHED();
+        m_scriptHasError = true;
         DartUtilities::reportProblem(m_originDocument, "Invalid import tag");
         return Dart_NewBoolean(false);
     }
@@ -260,6 +265,7 @@ Dart_Handle DartApplicationLoader::topLevelLibrary()
 
 void DartApplicationLoader::scriptLoadError(String failedUrl)
 {
+    m_scriptHasError = true;
     // FIXME: try to dig out line number, -1 for now.
     if (failedUrl.startsWith(String("dart:"))) {
         m_originDocument->logExceptionToConsole(String("The built-in library '") + failedUrl + String("' is not available on Dartium."), m_libraryUrl, -1, 0, 0);