Add support for multi-roots

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 1764e5b2bb3e2c31bac01a94fdfcff617e325faa..8af2fe1a474edea7f5f90cd85de4f8e5f9015003 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -80,7 +80,8 @@ class SourceLoader<L> extends Loader<L> {
   Future<Token> tokenize(SourceLibraryBuilder library,
       {bool suppressLexicalErrors: false}) async {
     Uri uri = library.fileUri;
-    if (uri == null || uri.scheme != "file") {
+    String scheme = uri?.scheme;
+    if (uri == null || (scheme != "file" && scheme != "multi-root")) {

[Siggi Cherem (dart-lang), 2017/07/01 04:09:57]

I'm not super happy with this part of the change, other ideas?

[Paul Berry, 2017/07/04 15:32:17]

Agreed, I'm not thrilled with this either.  A particular disadvantage of this
approach is that if the user uses a "multi-root" scheme in a compilation where
multi-root is disabled, source_loader will think the URI is fine, and then the
user will see an ArgumentError bubble up from PhysicalFileSystem and it will
look like they've hit a compiler bug :(

IMO it should be the responsibility of the fileSystem to know what kinds of URIs
are valid/invalid, not the source_loader (this includes checking for a null
URI).  So my preference would be to modify PhysicalFileSystem.entityForUri so
that if it's passed a bad or null URI, it returns a new kind of FileSystemEntity
which responds to any read/write request with some sort of "bad URI" exception. 
(That way the behavior for a bad URI will be similar to the behavior for a
non-existent file).  Once we've done that, we should be able to remove lines
83-86 from this file entirely.

[Siggi Cherem (dart-lang), 2017/07/05 22:48:30]

I like your suggestion. In dart2js we also have a `memory:` scheme we used for
unit testing, hiding this check behind the FileSystem will let us support that
use case as well.

I'll do the change on a separate CL though.

       return inputError(library.uri, -1, "Not found: ${library.uri}.");
     }