Initial cut for a development server

lib/src/codegen/js_codegen.dart

 // Copyright (c) 2015, 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 ddc.src.codegen.js_codegen;
 
 import 'dart:collection' show HashSet;
 import 'dart:io' show Directory, File;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
@@ skipping 1922 matching lines @@
     if (name != null) {
       // TODO(jmesserly): mark only uses the beginning of the span, but
       // we're required to pass this as a valid span.
       var end = _location(node.end);
       printer.mark(new SourceMapSpan(loc, end, name, isIdentifier: true));
     } else {
       printer.mark(loc);
     }
   }
 
-  SourceLocation _location(int offset) {
-    var lineInfo = unit.lineInfo.getLocation(offset);
-    return new SourceLocation(offset,
-        sourceUrl: uri,
-        line: lineInfo.lineNumber - 1,
-        column: lineInfo.columnNumber - 1);
-  }
+  SourceLocation _location(int offset) => locationForOffset(unit, uri, offset);
 
   Uri _makeRelativeUri(Uri src) {
     return new Uri(path: path.relative(src.path, from: outputDir));
   }
 
   void exitNode(JS.Node jsNode) {
     AstNode node = jsNode.sourceInformation;
     if (node is CompilationUnit) {
       unit = null;
       uri = null;
       return;
     }
     if (unit == null || node == null || node.offset == -1) return;
 
     // TODO(jmesserly): in many cases marking the end will be unncessary.
     printer.mark(_location(node.end));
   }
 
   String _getIdentifier(AstNode node) {
     if (node is SimpleIdentifier) return node.name;
     return null;
   }
 }