Cache output of dart2js compilations that went wrong on disk.

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

Index: sdk/lib/_internal/compiler/implementation/dart2js.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/dart2js.dart	(revision 35513)
+++ sdk/lib/_internal/compiler/implementation/dart2js.dart	(working copy)
@@ -365,11 +365,11 @@
 
   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 '