Add option to disable IR.

dart/sdk/lib/_internal/compiler/implementation/ir/ir_builder.dart

 // Copyright (c) 2013, 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 dart2js.ir_builder;
 
 import 'ir_nodes.dart';
 import '../elements/elements.dart';
 import '../elements/modelx.dart' show FunctionElementX;
 import '../dart2jslib.dart';
@@ skipping 77 matching lines @@
     });
   }
 
   bool irEnabled() {
     // TODO(lry): support checked-mode checks.
     if (compiler.enableTypeAssertions ||
         compiler.backend is !JavaScriptBackend ||
         compiler.enableConcreteTypeInference) {
       return false;
     }
-    return true;
+    return const bool.fromEnvironment('enable_ir', defaultValue: true);
   }
 
   bool canBuild(Element element) {
     // TODO(lry): support lazy initializers.
     FunctionElement function = element.asFunctionElement();
     if (function == null) return false;
 
     // TODO(lry): support functions with parameters.
     FunctionSignature signature = function.computeSignature(compiler);
     if (signature.parameterCount > 0) return false;
@@ skipping 306 matching lines @@
 
   void exitBlock() {
     blockBuilders.removeLast();
   }
 }
 
 class BlockBuilder {
   List<IrNode> statements = <IrNode>[];
   bool hasReturn = false;
 }