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) {

[floitsch, 2012/02/20 12:11:03]

&& receiver !== null

This should be more correct and more efficient.
If we have a 'this' (and that's when we don't have a receiver) then we know that
the Dart object exists and that we don't need to intercept.

[ahe, 2012/02/20 12:33:22]

Thank you, Florian. That makes sense. I assume you mean:

&& node.receiver !== null.

[floitsch, 2012/02/20 12:56:51]

correct.

       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());
+      if (node.receiver !== null) {
+        visit(node.receiver);
+        inputs.add(pop());
+      }
       addGenericSendArgumentsToList(node.arguments, inputs);
       push(new HInvokeInterceptor(selector, dartMethodName, false, inputs));
       return;
     }
 
     if (node.receiver === null) {
       inputs.add(localsHandler.readThis());
     } else {
       visit(node.receiver);
       inputs.add(pop());
@@ skipping 577 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     visit(node.block);
   }
 
   visitTypedef(Typedef node) {
     compiler.unimplemented('SsaBuilder.visitTypedef', node: node);
   }
 }