dart2js cps: Clean up and avoid processing unreachable code.

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

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index 25557a9ae81810308d223f8361a86f67da8eeda2..487eb4508fd26b79eba8879da264ec83b0baac8f 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -1075,12 +1075,6 @@ class CodeGenerator extends tree_ir.StatementVisitor
     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,