Fix this capturing in closures.

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 160 matching lines @@
       functionElement = body.constructor;
     }
     // If the element is not declared in the current function and the element
     // is not the closure itself we need to mark the element as free variable.
     if (element.enclosingElement != functionElement &&
         element != functionElement) {
       assert(closureData.freeVariableMapping[element] == null ||
              closureData.freeVariableMapping[element] == element);
       closureData.freeVariableMapping[element] = element;
     } else if (inTryCatchOrFinally) {
-      // TODO(ngeoffray): only do this if the variable is mutated.
-      closureData.usedVariablesInTry.add(element);
+      // Don't mark the this-element. This would complicate things in the
+      // builder.
+      if (element != closureData.thisElement) {
+        // TODO(ngeoffray): only do this if the variable is mutated.
+        closureData.usedVariablesInTry.add(element);
+      }
     }
   }
 
   void declareLocal(Element element) {
     scopeVariables.add(element);
   }
 
   visit(Node node) => node.accept(this);
 
   visitNode(Node node) => node.visitChildren(this);
@@ skipping 208 matching lines @@
     declareLocal(elements[node]);
   }
 
   visitTryStatement(TryStatement node) {
     // TODO(ngeoffray): implement finer grain state.
     inTryCatchOrFinally = true;
     node.visitChildren(this);
     inTryCatchOrFinally = false;
   }
 }