Provide URLs for assets from lib/ and asset/ in pub's websocket API.

sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart

Index: sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart b/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
index fe3c1b0e02029cfcb11c9afb05caaed3961213c1..0eee8e7d28b28d381ab72a52fc3a9d04a6f956e7 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
@@ -8,6 +8,7 @@ import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
+import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
 import 'package:stack_trace/stack_trace.dart';
 
@@ -212,12 +213,12 @@ class WebSocketApi {
     return result;
   }
 
-  /// Given an asset ID in the root package, returns the URLs served by pub
-  /// that can be used to access that asset.
+  /// Given a path on the filesystem, returns the URLs served by pub that can be
+  /// used to access asset found at that path.
   ///
   /// The command name is "pathToUrls" and it takes a "path" key (a native OS
-  /// path relative to the root directory of the entrypoint package) for the
-  /// path being mapped:
+  /// path which may be absolute or relative to the root directory of the
+  /// entrypoint package) for the path being mapped:
   ///
   ///     {
   ///       "command": "pathToUrls",
@@ -231,6 +232,26 @@ class WebSocketApi {
   ///       "urls": ["http://localhost:8080/index.html"]
   ///     }
   ///
+  /// The "path" key may refer to a path in another package, either by referring
+  /// to its location within the top-level "packages" directory or by referring
+  /// to its location on disk. Only the "lib" and "asset" directories are
+  /// visible in other packages:
+  ///
+  ///     {
+  ///       "command": "assetIdToUrl",
+  ///       "path": "packages/http/http.dart"
+  ///     }
+  ///
+  /// Assets in the "lib" and "asset" directories will usually have one URL for
+  /// each server:
+  ///
+  ///     {
+  ///       "urls": [
+  ///         "http://localhost:8080/packages/http/http.dart",
+  ///         "http://localhost:8081/packages/http/http.dart"
+  ///       ]
+  ///     }
+  ///
   /// An optional "line" key may be provided whose value must be an integer. If
   /// given, the result will also include a "line" key that maps the line in
   /// the source file to the corresponding output line in the resulting asset
@@ -245,32 +266,19 @@ class WebSocketApi {
   /// If the asset is not in a directory being served by pub, returns an error:
   ///
   ///     example/index.html  -> NOT_SERVED error
-  ///
-  /// This cannot currently be used to access assets in other packages aside
-  /// from the root. Nor can it be used to access assets in the root package's
-  /// "lib" or "asset" directories.
-  ///
-  ///     lib/myapp.dart  -> BAD_ARGUMENT error
   Map _pathToUrls(Map command) {
-    // TODO(rnystrom): Support assets in other packages. See #17146.
-    var assetPath = _validateRelativePath(command, "path");
+    var assetPath = _validateString(command, "path");
     var line = _validateOptionalInt(command, "line");
 
-    // Find all of the servers whose root directories contain the asset and
-    // generate appropriate URLs for each.
-    var urls = _environment.servers
-        .where((server) => path.isWithin(server.rootDirectory, assetPath))
-        .map((server) {
-      var relativePath = path.relative(assetPath, from: server.rootDirectory);
-      return "${server.url}/${path.toUri(relativePath)}";
-    }).toList();
-
+    var urls = _environment.getUrlsForAssetPath(assetPath);
     if (urls.isEmpty) {
+      var name = '"$assetPath"';
+      if (command['package'] != null) name += ' in package "$package"';
       throw new _WebSocketException(_ErrorCode.NOT_SERVED,
-          'Asset path "$assetPath" is not currently being served.');
+          'Asset path $name is not currently being served.');
     }
 
-    var result = {"urls": urls};
+    var result = {"urls": urls.map((url) => url.toString()).toList()};
 
     // Map the line.
     // TODO(rnystrom): Right now, source maps are not supported and it just
@@ -305,7 +313,7 @@ class WebSocketApi {
     var rootDirectory = _validateRelativePath(command, "path");
     return _environment.serveDirectory(rootDirectory).then((server) {
       return {
-        "url": server.url
+        "url": server.url.toString()
       };
     });
   }
@@ -314,14 +322,15 @@ class WebSocketApi {
   ///
   /// Returns the string if found, or throws a [_WebSocketException] if
   /// validation failed.
-  String _validateString(Map command, String key) {
-    if (!command.containsKey(key)) {
+  String _validateString(Map command, String key, {bool optional: false}) {
+    if (!optional && !command.containsKey(key)) {
       throw new _WebSocketException(_ErrorCode.BAD_ARGUMENT,
           'Missing "$key" argument.');
     }
 
     var field = command[key];
     if (field is String) return field;
+    if (field == null && optional) return null;
 
     throw new _WebSocketException(_ErrorCode.BAD_ARGUMENT,
         '"$key" must be a string. Got ${JSON.encode(field)}.');