(Fix #215) better error printing in editor & extension, adds mapping for editor

lib/component_build.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Common logic to make it easy to create a `build.dart` for your project.
  *
  * The `build.dart` script is invoked automatically by the Editor whenever a
  * file in the project changes. It must be placed in the root of a project
  * (where pubspec.yaml lives) and should be named exactly 'build.dart'.
@@ skipping 19 matching lines @@
  * live will be scanned for generated files to delete them.
  */
 // TODO(jmesserly): we need a better way to automatically detect input files
 void build(List<String> arguments, List<String> entryPoints) {
   var args = _processArgs(arguments);
 
   var trackDirs = <Directory>[];
   var changedFiles = args["changed"];
   var removedFiles = args["removed"];
   var cleanBuild = args["clean"];
+  var machineFormat = args["machine"];
   var fullBuild = changedFiles.isEmpty && removedFiles.isEmpty && !cleanBuild;
 
   for (var file in entryPoints) {
     trackDirs.add(new Directory(_outDir(file)));
   }
 
   if (cleanBuild) {
     _handleCleanCommand(trackDirs);
   } else if (fullBuild || changedFiles.some((f) => _isInputFile(f, trackDirs))
       || removedFiles.some((f) => _isInputFile(f, trackDirs))) {
-    // TODO(sigmund): allow colors if we find a way to detect reliably where
-    // colors are supported (e.g. we are not running in the editor or in
-    // windows) 
     for (var file in entryPoints) {
-      dwc.run(['--no-colors', '-o', _outDir(file), file]);
+      var path = new Path(file);
+      var outDir = path.directoryPath.append('out');
+      var args = [];
+      if (machineFormat) args.add('--json_format');
+      if (Platform.operatingSystem == 'windows') args.add('--no-colors');
+      args.addAll(['-o', outDir.toString(), file]);
+      dwc.run(args);
+
+      if (machineFormat) {
+        // Print for the Dart Editor the mapping from the input entry point file
+        // and its corresponding output.
+        var out = outDir.append(path.filename);
+        print('[{"method":"mapping","params":{"from":"$file","to":"$out"}}]');
+      }
     }
   }
 }
 
 String _outDir(String file) =>
   new Path(file).directoryPath.append('out').toString();
 
 bool _isGeneratedFile(String filePath, List<Directory> outputDirs) {
   var path = new Path.fromNative(filePath);
   for (var dir in outputDirs) {
@@ skipping 36 matching lines @@
     ..addFlag("machine", negatable: false,
         help: "produce warnings in a machine parseable format")
     ..addFlag("help", negatable: false, help: "displays this help and exit");
   var args = parser.parse(arguments);
   if (args["help"]) {
     print(parser.getUsage());
     exit(0);
   }
   return args;
 }