Mix in [TreeElementMixin] only on nodes that need it.

sdk/lib/_internal/compiler/implementation/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.
 
 library closureToClassMapper;
 
 import "elements/elements.dart";
 import "dart2jslib.dart";
 import "dart_types.dart";
 import "js_backend/js_backend.dart" show JavaScriptBackend;
@@ skipping 984 matching lines @@
       // the work of the resolver. It is the resolver's job to create
       // parameters, etc. Other phases should only visit statements.
       if (node.parameters != null) node.parameters.accept(this);
       if (node.initializers != null) node.initializers.accept(this);
       if (node.body != null) node.body.accept(this);
     });
   }
 
   visitFunctionDeclaration(FunctionDeclaration node) {
     node.visitChildren(this);
-    LocalFunctionElement localFunction = elements[node];
+    LocalFunctionElement localFunction =
+        elements.getFunctionDefinition(node.function);
     declareLocal(localFunction);
   }
 
   visitTryStatement(TryStatement node) {
     // TODO(ngeoffray): implement finer grain state.
     bool oldInTryStatement = inTryStatement;
     inTryStatement = true;
     node.visitChildren(this);
     inTryStatement = oldInTryStatement;
   }
 }
 
 /// A type variable as a local variable.
 class TypeVariableLocal implements Local {
   final TypeVariableType typeVariable;
   final ExecutableElement executableContext;
 
   TypeVariableLocal(this.typeVariable, this.executableContext);
 
   String get name => typeVariable.name;
 
   int get hashCode => typeVariable.hashCode;
 
   bool operator ==(other) {
     if (other is! TypeVariableLocal) return false;
     return typeVariable == other.typeVariable;
   }
 }