Cache output of dart2js compilations that went wrong on disk.

sdk/lib/_internal/compiler/implementation/dart2js.dart

 // Copyright (c) 2013, 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 dart2js.cmdline;
 
 import 'dart:async'
     show Future, EventSink;
 import 'dart:io'
     show exit, File, FileMode, Platform, RandomAccessFile, FileSystemException;
@@ skipping 347 matching lines @@
 
   int totalCharactersWritten = 0;
 
   options.add('--out=$out');
   options.add('--source-map=$sourceMapOut');
 
   List<String> allOutputFiles = new List<String>();
 
   compilationDone(String code) {
     if (analyzeOnly) return;
+    writeString(Uri.parse('$out.deps'),
+                getDepsOutput(inputProvider.sourceFiles));

[kustermann, 2014/04/29 21:20:44]

This does not work in a very rare edge case:

Assuming you have a dart test X.dart importing Y.dart. But Y.dart is not present
on the filesystem. In that case [inputProvider.sourceFiles] will only contain
'X.dart' but not 'Y.dart' and the compilation will fail.

Now if someone adds 'Y.dart', we don't rerun the dart2js compilation (since
'X.dart' and the sdk did not change) -- which is broken (since adding 'Y.dart'
could make the compilation succeed).

[ricow1, 2014/04/30 06:51:01]

I don't know enough about how dart2js works to decide on this, but here is a few
questions:
  By your example I assume that the files are only added after they have
actually been read in (or we validate that they are on disk? )
  If so, could we add them elsewhere so that we can combine missing input files
with the actual files and thereby eliminate this problem?

     if (code == null) {
       fail('Compilation failed.');
     }
-    writeString(Uri.parse('$out.deps'),
-                getDepsOutput(inputProvider.sourceFiles));
     diagnosticHandler.info(
          'Compiled ${inputProvider.dartCharactersRead} characters Dart '
          '-> $totalCharactersWritten characters $outputLanguage '
          'in ${relativize(currentDirectory, out, isWindows)}');
     if (diagnosticHandler.verbose) {
       String input = uriPathToNative(arguments[0]);
       print('Dart file ($input) compiled to $outputLanguage.');
       print('Wrote the following files:');
       for (String filename in allOutputFiles) {
         print("  $filename");
@@ skipping 280 matching lines @@
     }
   }
 
   try {
     return compilerMain(arguments).catchError(onError);
   } catch (exception, trace) {
     onError(exception, trace);
     return new Future.value();
   }
 }