Fix a crash in the compiler.

sdk/lib/_internal/compiler/implementation/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.
 
 part of ssa;
 
 /**
  * A special element for the extra parameter taken by intercepted
  * methods. We need to override [Element.computeType] because our
  * optimizers may look at its declared type.
@@ skipping 631 matching lines @@
       });
     }
     if (thisValue != null) {
       // If there was a "this" for the scope, add it to the new locals.
       joinedLocals[closureData.thisElement] = thisValue;
     }
 
     // Remove locals that are not in all handlers.
     directLocals = new Map<Element, HInstruction>();
     joinedLocals.forEach((element, instruction) {
-      if (instruction is HPhi
+      if (element != closureData.thisElement
           && instruction.inputs.length != localsHandlers.length) {
         joinBlock.removePhi(instruction);
       } else {
         directLocals[element] = instruction;
       }
     });
     return this;
   }
 }
 
@@ skipping 5326 matching lines @@
     DartType unaliased = type.unalias(builder.compiler);
     if (unaliased is TypedefType) throw 'unable to unalias $type';
     unaliased.accept(this, builder);
   }
 
   void visitDynamicType(DynamicType type, SsaBuilder builder) {
     ClassElement cls = builder.compiler.findHelper('DynamicRuntimeType');
     builder.push(new HDynamicType(type, new TypeMask.exact(cls)));
   }
 }