Remove check for scheme in source-loader.

pkg/front_end/lib/src/fasta/source/source_loader.dart

Index: pkg/front_end/lib/src/fasta/source/source_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_loader.dart b/pkg/front_end/lib/src/fasta/source/source_loader.dart
index 638598d4494f17fb86f20b74a180dbf487d1cbcd..c15da0140620f092b8d7e315ffc73954c4c13467 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -96,12 +96,6 @@ class SourceLoader<L> extends Loader<L> {
   Future<Token> tokenize(SourceLibraryBuilder library,
       {bool suppressLexicalErrors: false}) async {
     Uri uri = library.fileUri;
-    // TODO(sigmund): source-loader shouldn't check schemes, but defer to the
-    // underlying file system to decide whether it is supported.
-    if (uri == null || uri.scheme != "file" && uri.scheme != "multi-root") {

[Siggi Cherem (dart-lang), 2017/08/23 20:38:33]

note that the `fileSystem.entityForUri` call below will report an error when uri
is `null`. It is now up to the file system to define what is supported.

We could add a `fileSystem.exists(uri)` here as well if we want to produce a
different error, but it feels redundant.

[ahe, 2017/08/24 10:08:53]

The following situations are still erroneous at this point:

uri == null
!uri.hasScheme

Regardless if fileSystem can handle scheme-less URIs, that's a problem here as
it would imply that a library.uri didn't have a scheme. That would be a major
invariant violation that would cause problems elsewhere in Fasta.

Make sure that this doesn't crash:

./pkg/front_end/tool/fasta testing compile/rasta/unsupported_platform_library

[Siggi Cherem (dart-lang), 2017/08/24 18:55:19]

Done - as you suggested now I'm checking for null and !hasScheme.

The `null` case would have been handled by the filesystem regardless, but it's
fine to have it here. For the hasScheme I added a new branch so we can provide a
more informative error message. I switched to use internalProblem for it.

[ahe, 2017/08/25 08:52:31]

That's odd. When I ran the test with this code removed, it crashed. Also, I'm
not sure how FileSystem would be able to diagnose that "dart:html" is missing
based on being passed null?
Great idea.

-      return deprecated_inputError(
-          library.uri, -1, "Not found: ${library.uri}.");
-    }
 
     // Get the library text from the cache, or read from the file system.
     List<int> bytes = sourceBytes[uri];