Add support for multi-roots

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

 // Copyright (c) 2016, 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 fasta.source_loader;
 
 import 'dart:async' show Future;
 
 import 'dart:typed_data' show Uint8List;
 
@@ skipping 62 matching lines @@
 
   TypeInferenceEngine typeInferenceEngine;
 
   Instrumentation instrumentation;
 
   SourceLoader(this.fileSystem, KernelTarget target) : super(target);
 
   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}.");
     }
 
     // Get the library text from the cache, or read from the file system.
     List<int> bytes = sourceBytes[uri];
     if (bytes == null) {
       try {
         List<int> rawBytes = await fileSystem.entityForUri(uri).readAsBytes();
         Uint8List zeroTerminatedBytes = new Uint8List(rawBytes.length + 1);
         zeroTerminatedBytes.setRange(0, rawBytes.length, rawBytes);
@@ skipping 418 matching lines @@
 
   Expression throwCompileConstantError(Expression error) {
     return target.backendTarget.throwCompileConstantError(coreTypes, error);
   }
 
   Expression buildCompileTimeError(String message, int offset) {
     return target.backendTarget
         .buildCompileTimeError(coreTypes, message, offset);
   }
 }