Fix crash in SSA builder for receiver-less calls. Not sure this is correct.

dart/frog/leg/ssa/builder.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 Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 1478 matching lines @@
       isNotEquals = name.stringValue === '!=';
       dartMethodName = Elements.constructOperatorName(
           const SourceString('operator'),
           name,
           node.argumentsNode is Prefix);
     } else {
       dartMethodName = node.selector.asIdentifier().source;
     }
 
     Element interceptor = null;
-    if (methodInterceptionEnabled) {
+    if (methodInterceptionEnabled && node.receiver !== null) {

[ngeoffray, 2012/02/20 17:14:08]

Maybe it would be clearer to check if you actually have an element for that
call. If you have an element, you know it should not be intercepted.

Also, whatever solution, this fix deserves a comment.

       interceptor = interceptors.getStaticInterceptor(dartMethodName,
                                                       node.argumentCount());
     }
     if (interceptor != null) {
       HStatic target = new HStatic(interceptor);
       add(target);
       inputs.add(target);
       visit(node.receiver);
       inputs.add(pop());
       addGenericSendArgumentsToList(node.arguments, inputs);
@@ skipping 586 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     visit(node.block);
   }
 
   visitTypedef(Typedef node) {
     compiler.unimplemented('SsaBuilder.visitTypedef', node: node);
   }
 }