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

dart/frog/leg/ssa/builder.dart

Index: dart/frog/leg/ssa/builder.dart
diff --git a/dart/frog/leg/ssa/builder.dart b/dart/frog/leg/ssa/builder.dart
index 40c6d540a7142671fb5e556e4eca931f868a6442..473c820bb59a52cf1d65e6703f23aecf37df746a 100644
--- a/dart/frog/leg/ssa/builder.dart
+++ b/dart/frog/leg/ssa/builder.dart
@@ -1496,7 +1496,7 @@ class SsaBuilder implements Visitor {
     }
 
     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());
     }