Fix this capturing in closures.

frog/leg/ssa/builder.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 Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 246 matching lines @@
     closureData = translator.translate(node);
 
     FunctionParameters params = function.computeParameters(builder.compiler);
     params.forEachParameter((Element element) {
       HParameterValue parameter = new HParameterValue(element);
       builder.add(parameter);
       // Note that for constructors [element] could be a field-element which we
       // 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);
 
     // If the freeVariableMapping is not empty, then this function was a
     // nested closure that captures variables. Redirect the captured
     // variables to fields in the closure.
     closureData.freeVariableMapping.forEach((Element from, Element to) {
       redirectElement(from, to);
     });
     if (closureData.isClosure()) {
       // Inside closure redirect references to itself to [:this:].
       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);
     }
   }
 
   bool hasValueForDirectLocal(Element element) {
     assert(element !== null);
     assert(isAccessedDirectly(element));
     return directLocals[element] !== null;
   }
 
   /**
@@ skipping 2042 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     visit(node.block);
   }
 
   visitTypedef(Typedef node) {
     compiler.unimplemented('SsaBuilder.visitTypedef', node: node);
   }
 }