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

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

Index: sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart b/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
index 99b3919556d7d4a30415c24eaa59a2063c2f5447..e58280af1bf3b3112ee1e978e9821481ee78bc80 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
@@ -162,6 +162,61 @@ class BuildEnvironment {
     });
   }
 
+  /// Return all URLs serving [assetPath] in this environment.
+  List<Uri> getUrlsForAssetPath(String assetPath) {
+    // Check the three (mutually-exclusive) places the path could be pointing.
+    var urls = _lookUpPathInServerRoot(assetPath);
+    if (urls.isEmpty) urls = _lookUpPathInPackagesDirectory(assetPath);
+    if (urls.isEmpty) urls = _lookUpPathInDependency(assetPath);
+    return urls.toList();
+  }
+
+  /// Look up [assetPath] in the root directories of servers running in the
+  /// entrypoint package.
+  Iterable<Uri> _lookUpPathInServerRoot(String assetPath) {
+    // Find all of the servers whose root directories contain the asset and
+    // generate appropriate URLs for each.
+    return servers
+        .where((server) => path.isWithin(server.rootDirectory, assetPath))
+        .map((server) {
+      var relativePath = path.relative(assetPath, from: server.rootDirectory);
+      return server.url.resolveUri(path.toUri(relativePath));
+    });
+  }
+
+  /// Look up [assetPath] in the "packages" directory in the entrypoint package.
+  Iterable<Uri> _lookUpPathInPackagesDirectory(String assetPath) {
+    var components = path.split(path.relative(assetPath));
+    if (components.first != "packages") return [];
+    if (!graph.packages.containsKey(components[1])) return [];
+    return servers.map((server) =>
+        server.url.resolveUri(path.toUri(assetPath)));
+  }
+
+  /// Look up [assetPath] in the "lib" or "asset" directory of a dependency
+  /// package.
+  Iterable<Uri> _lookUpPathInDependency(String assetPath) {
+    for (var package in graph.packages.values) {
+      var libDir = path.join(package.dir, 'lib');
+      var assetDir = path.join(package.dir, 'asset');
+
+      var uri;
+      if (path.isWithin(libDir, assetPath)) {
+        uri = path.toUri(path.join('packages', package.name,
+            path.relative(assetPath, from: libDir)));
+      } else if (path.isWithin(assetDir, assetPath)) {
+        uri = path.toUri(path.join('assets', package.name,
+            path.relative(assetPath, from: assetDir)));
+      } else {
+        continue;
+      }
+
+      return servers.map((server) => server.url.resolveUri(uri));
+    }
+
+    return [];
+  }
+
   /// Determines if [sourcePath] is contained within any of the directories
   /// that are visible to this build environment.
   bool containsPath(String sourcePath) {