Move helpers and interceptors to own library.

dart/frog/leg/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";
 
   void check(Node tree, TreeElements elements) {
     measure(() {
@@ skipping 357 matching lines @@
         ClassElement classElement = receiverType.element;
         // TODO(karlklose): substitute type arguments.
         Type memberType = lookupMethodType(node, classElement, selector.source);
         if (memberType === types.dynamicType) return types.dynamicType;
         if (memberType is !FunctionType) {
           fail(node, 'can only handle function types');
         }
         funType = memberType;
       } else {
         Element element = elements[node];
-        if (element.kind === ElementKind.FUNCTION) {
+        if (element === null) {
+          fail(node, 'unresolved ${node.selector}');
+        } else if (element.kind === ElementKind.FUNCTION) {
           funType = computeType(element);
         } else if (element.kind === ElementKind.FOREIGN) {
           return types.dynamicType;
         } else {
           fail(node, 'unexpected element kind ${element.kind}');
         }
       }
       Link<Type> parameterTypes = funType.parameterTypes;
       Link<Node> argumentNodes = node.arguments;
       while (!argumentTypes.isEmpty() && !parameterTypes.isEmpty()) {
@@ skipping 236 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     compiler.unimplemented('visitCatchBlock', node: node);
   }
 
   visitTypedef(Typedef node) {
     compiler.unimplemented('visitTypedef', node: node);
   }
 }