Fix warnings in dart2js.

sdk/lib/_internal/compiler/implementation/resolution/members.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.
 
 part of resolution;
 
 abstract class TreeElements {
   Element get currentElement;
   Setlet<Node> get superUses;
 
@@ skipping 443 matching lines @@
         return new TreeElementMapping(element);
       }
       if (element.isPatched) {
         checkMatchingPatchSignatures(element, element.patch);
         element = element.patch;
       }
       return compiler.withCurrentElement(element, () {
         FunctionExpression tree = element.parseNode(compiler);
         if (tree.modifiers.isExternal()) {
           error(tree, MessageKind.PATCH_EXTERNAL_WITHOUT_IMPLEMENTATION);
-          return;
+          return null;
         }
         if (isConstructor || element.isFactoryConstructor()) {
           if (tree.returnType != null) {
             error(tree, MessageKind.CONSTRUCTOR_WITH_RETURN_TYPE);
           }
           if (element.modifiers.isConst() &&
               tree.hasBody() &&
               !tree.isRedirectingFactory) {
             compiler.reportError(tree, MessageKind.CONST_CONSTRUCTOR_HAS_BODY);
           }
@@ skipping 2120 matching lines @@
       }
     }
 
     if (!resolvedArguments) {
       resolveArguments(node.argumentsNode);
     }
 
     // If the selector is null, it means that we will not be generating
     // code for this as a send.
     Selector selector = mapping.getSelector(node);
-    if (selector == null) return;
+    if (selector == null) return null;
 
     if (node.isCall) {
       if (Elements.isUnresolved(target) ||
           target.isGetter() ||
           target.isField() ||
           Elements.isClosureSend(node, target)) {
         // If we don't know what we're calling or if we are calling a getter,
         // we need to register that fact that we may be calling a closure
         // with the same arguments.
         Selector call = new Selector.callClosureFrom(selector);
@@ skipping 2065 matching lines @@
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }
 
 /// Looks up [name] in [scope] and unwraps the result.
 Element lookupInScope(Compiler compiler, Node node,
                       Scope scope, String name) {
   return Elements.unwrap(scope.lookup(name), compiler, node);
 }