Fix leg assert and NPE to make BB green.

frog/leg/ssa/closure.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 ClosureFieldElement extends Element {
   ClosureFieldElement(SourceString name, ClassElement enclosing)
       : super(name, ElementKind.FIELD, enclosing);
 
   bool isInstanceMember() => true;
   bool isAssignable() => false;
@@ skipping 378 matching lines @@
     scopeVariables = oldScopeVariables;
     insideClosure = oldInsideClosure;
     closureData = oldClosureData;
     currentFunctionElement = oldFunctionElement;
 
     // Mark all free variables as captured and use them in the outer function.
     List<Element> freeVariables =
         savedClosureData.freeVariableMapping.getKeys();
     assert(freeVariables.isEmpty() || savedInsideClosure);
     for (Element freeElement in freeVariables) {
-      assert(capturedVariableMapping[freeElement] == null ||
-             capturedVariableMapping[freeElement] == freeElement);
+      if (capturedVariableMapping[freeElement] != null &&
+          capturedVariableMapping[freeElement] != freeElement) {
+        compiler.internalError('In closure analyzer', node: node);
+      }
       capturedVariableMapping[freeElement] = freeElement;
       useLocal(freeElement);
     }
   }
 
   visitFunctionDeclaration(FunctionDeclaration node) {
     node.visitChildren(this);
     declareLocal(elements[node]);
   }
 
   visitTryStatement(TryStatement node) {
     // TODO(ngeoffray): implement finer grain state.
     inTryCatchOrFinally = true;
     node.visitChildren(this);
     inTryCatchOrFinally = false;
   }
 }