Add type annotations to remove warnings and fix a bug.

lib/compiler/implementation/typechecker.dart

 // 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.
 
 class TypeCheckerTask extends CompilerTask {
   TypeCheckerTask(Compiler compiler) : super(compiler);
   String get name => "Type checker";
 
   static const bool LOG_FAILURES = false;
 
@@ skipping 971 matching lines @@
     compiler.unimplemented('visitNode', node: node);
   }
 
   DartType visitStatement(Statement node) {
     compiler.unimplemented('visitNode', node: node);
   }
 
   DartType visitStringNode(StringNode node) {
     compiler.unimplemented('visitNode', node: node);
   }
+
+  DartType visitLibraryDependency(LibraryDependency node) {
+    compiler.unimplemented('visitNode', node: node);
+  }
 }