Fix this capturing in closures.

frog/leg/ssa/builder.dart

Index: frog/leg/ssa/builder.dart
diff --git a/frog/leg/ssa/builder.dart b/frog/leg/ssa/builder.dart
index c7da42a91e453aefc16f78f688cbdc7ccefca0b6..a04617bd7dea5271c08f65cb410051d5374aeebf 100644
--- a/frog/leg/ssa/builder.dart
+++ b/frog/leg/ssa/builder.dart
@@ -264,6 +264,18 @@ class LocalsHandler {
       // treat as if it was a local.
       directLocals[element] = parameter;
     });
+    if (closureData.thisElement !== null) {
+      // Once closures have been mapped to classes their instance members might
+      // not have any thisElement if the closure was created inside a static
+      // context.
+      assert(function.isInstanceMember() || function.isGenerativeConstructor());
+      // We have to introduce 'this' before we enter the scope, since it might
+      // need to be copied into a box (if it is captured). This is similar
+      // to all other parameters that are introduced.
+      HInstruction thisInstruction = new HThis();
+      builder.add(thisInstruction);
+      directLocals[closureData.thisElement] = thisInstruction;
+    }
 
     enterScope(node);
 
@@ -278,11 +290,6 @@ class LocalsHandler {
       HInstruction thisInstruction = new HThis();
       builder.add(thisInstruction);
       updateLocal(closureData.closureElement, thisInstruction);
-    } else if (function.isInstanceMember() ||
-               function.isGenerativeConstructor()) {
-      HInstruction thisInstruction = new HThis();
-      builder.add(thisInstruction);
-      updateLocal(closureData.thisElement, thisInstruction);
     }
   }