dart2js cps: Clean up and avoid processing unreachable code.

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 '../../closure.dart' show
     ClosureClassElement;
@@ skipping 1057 matching lines @@
     js.Expression receiver = visitExpression(node.receiver);
     List<js.Expression> args = visitExpressionList(node.arguments);
     return js.js('#.#(#)', [receiver, name, args]);
   }
 
   @override
   js.Expression visitAwait(tree_ir.Await node) {
     return new js.Await(visitExpression(node.input));
   }
 
-  visitFunctionExpression(tree_ir.FunctionExpression node) {
-    // FunctionExpressions are currently unused.
-    // We might need them if we want to emit raw JS nested functions.
-    throw 'FunctionExpressions should not be used';
-  }
-
   /// Ensures that parameter defaults will be emitted.
   ///
   /// Ideally, this should be done when generating the relevant stub methods,
   /// since those are the ones that actually reference the constants, but those
   /// are created by the emitter when it is too late to register new constants.
   ///
   /// For non-static methods, we have no way of knowing if the defaults are
   /// actually used, so we conservatively register them all.
   void registerDefaultParameterValues(ExecutableElement element) {
     if (element is! FunctionElement) return;
     FunctionElement function = element;
     if (function.isStatic) return; // Defaults are inlined at call sites.
     function.functionSignature.forEachOptionalParameter((param) {
       ConstantValue constant = glue.getDefaultParameterValue(param);
       registry.registerCompileTimeConstant(constant);
     });
   }
 }