Use URIs rather than paths in front end API.

pkg/front_end/lib/src/base/processed_options.dart

Index: pkg/front_end/lib/src/base/processed_options.dart
diff --git a/pkg/front_end/lib/src/base/processed_options.dart b/pkg/front_end/lib/src/base/processed_options.dart
index e1a4ef4dcba72e84d6bc9f71f7b25b8c2c3a905e..ba676513579993a56b77861062c31bb3963d7901 100644
--- a/pkg/front_end/lib/src/base/processed_options.dart
+++ b/pkg/front_end/lib/src/base/processed_options.dart
@@ -39,11 +39,11 @@ class ProcessedOptions {
   /// Get the [FileSystem] which should be used by the front end to access
   /// files.
   ///
-  /// If the client supplied bazel roots using [CompilerOptions.bazelRoots], the
+  /// If the client supplied roots using [CompilerOptions.multiRoots], the
   /// returned [FileSystem] will automatically perform the appropriate mapping.
   FileSystem get fileSystem {
-    // TODO(paulberry): support bazelRoots.
-    assert(_raw.bazelRoots.isEmpty);
+    // TODO(paulberry): support multiRoots.
+    assert(_raw.multiRoots.isEmpty);
     return _raw.fileSystem;
   }
 
@@ -70,13 +70,12 @@ class ProcessedOptions {
     if (_packages == null) {
       if (_raw.packagesFilePath == null) {
         throw new UnimplementedError(); // TODO(paulberry): search for .packages
-      } else if (_raw.packagesFilePath.isEmpty) {
+      } else if (_raw.packagesFilePath.path.isEmpty) {
         _packages = {};
       } else {
         var contents =
-            await fileSystem.entityForPath(_raw.packagesFilePath).readAsBytes();
-        var baseLocation = fileSystem.context.toUri(_raw.packagesFilePath);
-        _packages = package_config.parse(contents, baseLocation);
+            await fileSystem.entityForUri(_raw.packagesFilePath).readAsBytes();
+        _packages = package_config.parse(contents, _raw.packagesFilePath);
       }
     }
     return _packages;