Fix another closure/type variable error by using analyzeTypeVariables.

sdk/lib/_internal/compiler/implementation/closure.dart

Index: sdk/lib/_internal/compiler/implementation/closure.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/closure.dart	(revision 28989)
+++ sdk/lib/_internal/compiler/implementation/closure.dart	(working copy)
@@ -555,11 +555,10 @@
     Element element = elements[node];
     if (Elements.isLocal(element)) {
       mutatedVariables.add(element);
+      if (compiler.enableTypeAssertions) {
+        analyzeTypeVariables(element.computeType(compiler));
+      }
     }
-    if (Elements.isLocal(element) &&
-        element.computeType(compiler).containsTypeVariables) {
-      registerNeedsThis();
-    }
     super.visitSendSet(node);
   }