Add support for SDK outline in KernelDriver.

pkg/front_end/lib/src/incremental/file_state.dart

Index: pkg/front_end/lib/src/incremental/file_state.dart
diff --git a/pkg/front_end/lib/src/incremental/file_state.dart b/pkg/front_end/lib/src/incremental/file_state.dart
index f6cc92ef3cf17427fee9bb8f06d4ae9e02845786..64ff7cd372b28b34bd62f02b37360a94d0864cfa 100644
--- a/pkg/front_end/lib/src/incremental/file_state.dart
+++ b/pkg/front_end/lib/src/incremental/file_state.dart
@@ -281,6 +281,10 @@ class FileSystemState {
   /// contain `file:*` URIs as keys.
   final Map<Uri, FileState> _fileUriToFile = {};
 
+  /// If `true`, then files with the `dart` scheme should be skipped.
+  /// We do this when we use SDK outline instead of compiling SDK sources.
+  bool skipSdkLibraries = false;
+
   FileSystemState(this._byteStore, this.fileSystem, this.uriTranslator,
       this._salt, this._newFileFn);
 
@@ -333,6 +337,12 @@ class FileSystemState {
   ///
   /// The returned file has the last known state since it was last refreshed.
   Future<FileState> getFile(Uri absoluteUri) async {
+    // We don't need to process SDK libraries if we have SDK outline.
+    // TODO(scheglov): Skip all libraries, or only included into the outline?
+    if (skipSdkLibraries && absoluteUri.isScheme('dart')) {
+      return null;
+    }
+
     // Resolve the absolute URI into the absolute file URI.
     Uri fileUri;
     if (absoluteUri.isScheme('file')) {