fix #620, infer the input files from sources

lib/src/compiler/code_generator.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.
 
 import 'dart:collection' show HashMap, HashSet;
 import 'dart:math' show min, max;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart' show Token, TokenType;
@@ skipping 218 matching lines @@
       var library = unit.element.library;
       if (unit.element != library.definingCompilationUnit) continue;
 
       var libraryTemp = _isDartRuntime(library)
           ? _runtimeLibVar
           : new JS.TemporaryId(jsLibraryName(_libraryRoot, library));
       _libraries[library] = libraryTemp;
       items.add(new JS.ExportDeclaration(
           js.call('const # = Object.create(null)', [libraryTemp])));
 
-      // dart:_runtime has a magic module that holds extenstion method symbols.
+      // dart:_runtime has a magic module that holds extension method symbols.
       // TODO(jmesserly): find a cleaner design for this.
       if (_isDartRuntime(library)) {
         items.add(new JS.ExportDeclaration(
             js.call('const # = Object.create(null)', [_dartxVar])));
       }
     }
 
     // Collect all Element -> Node mappings, in case we need to forward declare
     // any nodes.
     var nodes = new HashMap<Element, AstNode>.identity();
@@ skipping 5117 matching lines @@
 }
 
 bool isLibraryPrefix(Expression node) =>
     node is SimpleIdentifier && node.staticElement is PrefixElement;
 
 LibraryElement _getLibrary(AnalysisContext c, String uri) =>
     c.computeLibraryElement(c.sourceFactory.forUri(uri));
 
 bool _isDartRuntime(LibraryElement l) =>
     l.isInSdk && l.source.uri.toString() == 'dart:_runtime';