make build_sdk more incremental

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 155 matching lines @@
       checker.visitCompilationUnit(unit);
       if (checker.failure) failureInLib = true;
     }
 
     if (failureInLib) {
       _failure = true;
       if (!options.codegenOptions.forceCompile) return;
     }
 
     if (_jsGen != null) {
+      // TODO(jmesserly): full incremental support would avoid checking as well,

[Jennifer Messerly, 2015/08/31 23:43:08]

I'm already making good progress on addressing this.
But wanted to get the simpler variant out first.

+      // however, we'd lose compiler messages in that case.
+
+      // Note: analyzer's modification stamp is millisecondsSinceEpoch
+      int lastModifyTime = unitElements
+          .map((e) => context.getModificationStamp(e.source))
+          .reduce(math.max);
+      var outFile = new File(getOutputPath(library.source.uri));
+      if (outFile.existsSync() &&
+          outFile.lastModifiedSync().millisecondsSinceEpoch >= lastModifyTime) {
+        // Output already up to date.
+        return;
+      }
+
       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) {
@@ skipping 268 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');