dart2js cps: Remove dart2dart from cps pipeline and clean up.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

 // Copyright (c) 2014, 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 code_generator;
 
 import 'glue.dart';
 
 import '../../tree_ir/tree_ir_nodes.dart' as tree_ir;
 import '../../js/js.dart' as js;
@@ skipping 665 matching lines @@
     }
   }
 
   @override
   js.Expression visitTypeExpression(tree_ir.TypeExpression node) {
     List<js.Expression> arguments =
         node.arguments.map(visitExpression).toList(growable: false);
     return glue.generateTypeRepresentation(node.dartType, arguments);
   }
 
-  // Dart-specific IR nodes
-
-  @override
-  visitReifyTypeVar(tree_ir.ReifyTypeVar node) {
-    return errorUnsupportedNode(node);
-  }
-
-  @override
   visitFunctionExpression(tree_ir.FunctionExpression node) {
-    return errorUnsupportedNode(node);
-  }
-
-  @override
-  visitFunctionDeclaration(tree_ir.FunctionDeclaration node) {
-    return errorUnsupportedNode(node);
-  }
-
-  @override
-  visitVariableDeclaration(tree_ir.VariableDeclaration node) {
-    return errorUnsupportedNode(node);
-  }
-
-  errorUnsupportedNode(tree_ir.DartSpecificNode node) {
-    throw "Unsupported node in JS backend: $node";
+    // FunctionExpressions are currently unused.
+    // We might need them if we want to emit raw JS nested functions.
+    throw 'FunctionExpressions should not be used';
   }
 }