Fix warnings in dart2js.

sdk/lib/_internal/compiler/implementation/resolution/members.dart

Index: sdk/lib/_internal/compiler/implementation/resolution/members.dart
diff --git a/sdk/lib/_internal/compiler/implementation/resolution/members.dart b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
index bab8bfe1723144fd74b2153eed956a856b015106..015402a3b49b7bc5a7b69fcfec5208f8db817a64 100644
--- a/sdk/lib/_internal/compiler/implementation/resolution/members.dart
+++ b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
@@ -461,7 +461,7 @@ class ResolverTask extends CompilerTask {
         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) {
@@ -2602,7 +2602,7 @@ class ResolverVisitor extends MappingVisitor<Element> {
     // 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) ||