makes tests faster, see #304

lib/src/compiler.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.
 
 /// Command line tool to run the checker on a Dart program.
 library dev_compiler.src.compiler;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:math' as math;
@@ skipping 60 matching lines @@
   return new BatchCompiler(context, options, reporter: reporter).run();
 }
 
 class BatchCompiler extends AbstractCompiler {
   JSGenerator _jsGen;
   LibraryElement _dartCore;
   String _runtimeOutputDir;
 
   /// Already compiled sources, so we don't compile them again.
   final _compiled = new HashSet<LibraryElement>();
+  bool _sdkCopied = false;
 
   bool _failure = false;
   bool get failure => _failure;
 
   BatchCompiler(AnalysisContext context, CompilerOptions options,
       {AnalysisErrorListener reporter})
       : super(
             context,
             options,
             new ErrorCollector(
                 reporter ?? AnalysisErrorListener.NULL_LISTENER)) {
     _inputBaseDir = options.inputBaseDir;
     if (outputDir != null) {
       _jsGen = new JSGenerator(this);
       _runtimeOutputDir = path.join(outputDir, 'dev_compiler', 'runtime');
     }
     _dartCore = context.typeProvider.objectType.element.library;
   }
 
   ErrorCollector get reporter => checker.reporter;
 
   void reset() {
     _compiled.clear();
+    _sdkCopied = false;
   }
 
   /// Compiles every file in [options.inputs].
   /// Returns true on successful compile.
   bool run() {
     var clock = new Stopwatch()..start();
     options.inputs.forEach(compileFromUriString);
     clock.stop();
     var time = (clock.elapsedMilliseconds / 1000).toStringAsFixed(2);
     _log.fine('Compiled ${_compiled.length} libraries in ${time} s\n');
@@ skipping 57 matching lines @@
     }
 
     if (_jsGen != null) {
       var unit = units.first;
       var parts = units.skip(1).toList();
       _jsGen.generateLibrary(new LibraryUnit(unit, parts));
     }
   }
 
   void _copyDartRuntime() {
+    if (_sdkCopied) return;
+    _sdkCopied = true;
     for (var file in defaultRuntimeFiles) {
-      var input = path.join(options.runtimeDir, file);
-      var output = path.join(_runtimeOutputDir, file);
-      new Directory(path.dirname(output)).createSync(recursive: true);
-      new File(input).copySync(output);
+      var input = new File(path.join(options.runtimeDir, file));
+      var output = new File(path.join(_runtimeOutputDir, file));
+      if (output.existsSync() &&
+          output.lastModifiedSync() == input.lastModifiedSync()) {
+        continue;
+      }
+      new Directory(path.dirname(output.path)).createSync(recursive: true);
+      input.copySync(output.path);
     }
   }
 
   void _compileHtml(Source source) {
     // TODO(jmesserly): reuse DartScriptsTask instead of copy/paste.
     var contents = context.getContents(source);
     var document = html.parse(contents.data, generateSpans: true);
     var scripts = document.querySelectorAll('script[type="application/dart"]');
 
     var loadedLibs = new LinkedHashSet<Uri>();
@@ skipping 250 matching lines @@
     '_rtti.js',
     '_classes.js',
     '_operations.js',
     'dart_runtime.js',
   ];
   files.addAll(corelibOrder.map((l) => l.replaceAll('.', '/') + '.js'));
   return files;
 }();
 
 final _log = new Logger('dev_compiler.src.compiler');