cps-ir: Split up main codegen and add a bailout exception to print out

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 '../../tree_ir/tree_ir_nodes.dart' as tree_ir;
 import '../../js/js.dart' as js;
 import '../../elements/elements.dart';
 import '../../util/maplet.dart';
 
+class CodegenBailout {
+  final tree_ir.Node node;
+  final String reason;
+  CodegenBailout(this.node, this.reason);
+  String get message {
+    return 'bailout${node != null ? " on $node" : ""}: $reason';
+  }
+}
+
 class CodeGenerator extends tree_ir.Visitor<dynamic, js.Expression> {
-  static const String UNIMPLEMENTED = "Javascript generation aborted";
-
   /// Variables to be hoisted at the top of the current function.
   List<js.VariableDeclaration> variables = <js.VariableDeclaration>[];
 
   /// Maps variables to their name.
   Map<tree_ir.Variable, String> variableNames = <tree_ir.Variable, String>{};
 
   /// Maps local constants to their name.
   Maplet<VariableElement, String> constantNames =
       new Maplet<VariableElement, String>();
 
@@ skipping 15 matching lines @@
     for (tree_ir.Variable parameter in function.parameters) {
       parameters.add(new js.Parameter(variableNames[parameter]));
     }
     body = new js.Block(accumulator);
   }
 
   List<js.Expression> visitArguments(List<tree_ir.Expression> arguments) {
     return arguments.map(visitExpression).toList();
   }
 
-  giveup(tree_ir.Node node) {
-    throw UNIMPLEMENTED;
+  giveup(tree_ir.Node node,
+         [String reason = 'unimplemented in CodeGenerator']) {
+    throw new CodegenBailout(node, reason);
   }
 
   @override
   js.Expression visitConcatenateStrings(tree_ir.ConcatenateStrings node) {
     return giveup(node);
     // TODO: implement visitConcatenateStrings
   }
 
   @override
   js.Expression visitConditional(tree_ir.Conditional node) {
@@ skipping 140 matching lines @@
     // TODO: implement visitWhileTrue
   }
 
   @override
   void visitReturn(tree_ir.Return node) {
     if (node.value != null) {
       accumulator.add(new js.Return(visitExpression(node.value)));
     }
   }
 }