[dart2js] Add back kernel source file caching guarded by a flag.

Removing this cache means that errors from dill-loaded sources only display an offset rather than an exact file location. This makes debugging externally reported issues more difficult.

See here for more background: https://github.com/dart-lang/sdk/issues/52020#issuecomment-1513533855

Change-Id: I243d0bd340708ddbf979d77b5e9559751ee1b79e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/296040
Commit-Queue: Nate Biggs <natebiggs@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

pkg/compiler/lib/src/commandline_options.dart

@@ -190,6 +190,9 @@ class Flags {
 
   static const String cfeConstants = '--cfe-constants';
 
+  static const String disableDiagnosticByteCache =
+      '--disable-diagnostic-byte-cache';
+
   // `--no-shipping` and `--canary` control sets of flags. For simplicity, these
   // flags live in options.dart.
   // Shipping features default to on, but can be disabled individually. All

pkg/compiler/lib/src/dart2js.dart

@@ -685,6 +685,7 @@ Future<api.CompilationResult> compile(List<String> argv,
     _OneOption('${Flags.cfeInvocationModes}=.+', passThrough),
     _OneOption('${Flags.invoker}=.+', setInvoker),
     _OneOption('${Flags.verbosity}=.+', passThrough),
+    _OneOption(Flags.disableDiagnosticByteCache, passThrough),
 
     // Experimental features.
     // We don't provide documentation for these yet.
@@ -979,12 +980,16 @@ Future<api.CompilationResult> compile(List<String> argv,
           'The options --bazel-root and --multi-root cannot be supplied '
           'together, please choose one or the other.');
     }
-    inputProvider = BazelInputProvider(bazelPaths!, byteReader);
+    inputProvider = BazelInputProvider(bazelPaths!, byteReader,
+        disableByteCache: compilerOptions.disableDiagnosticByteCache);
   } else if (multiRoots != null) {
-    inputProvider =
-        MultiRootInputProvider(multiRootScheme!, multiRoots!, byteReader);
+    inputProvider = MultiRootInputProvider(
+        multiRootScheme!, multiRoots!, byteReader,
+        disableByteCache: compilerOptions.disableDiagnosticByteCache);
   } else {
-    inputProvider = CompilerSourceFileProvider(byteReader: byteReader);
+    inputProvider = CompilerSourceFileProvider(
+        byteReader: byteReader,
+        disableByteCache: compilerOptions.disableDiagnosticByteCache);
   }
 
   diagnostic.registerFileProvider(inputProvider);

pkg/compiler/lib/src/options.dart

@@ -599,6 +599,9 @@ class CompilerOptions implements DiagnosticOptions {
   // Whether or not to dump a list of unused libraries.
   bool dumpUnusedLibraries = false;
 
+  // Whether or not to disable byte cache for sources loaded from Kernel dill.
+  bool disableDiagnosticByteCache = false;
+
   late FeatureOptions features;
 
   // -------------------------------------------------
@@ -737,6 +740,8 @@ class CompilerOptions implements DiagnosticOptions {
           _extractStringOption(
               options, '${Flags.verbosity}=', fe.Verbosity.defaultValue)!,
           onError: onError)
+      ..disableDiagnosticByteCache =
+          _hasOption(options, Flags.disableDiagnosticByteCache)
       ..features = featureOptions;
   }
 

pkg/compiler/lib/src/source_file_provider.dart

@@ -25,8 +25,10 @@ abstract class SourceFileProvider implements api.CompilerInput {
   SourceFileByteReader byteReader;
   final Set<Uri> _registeredUris = {};
   final Map<Uri, Uri> _mappedUris = {};
+  final bool disableByteCache;
+  final Map<Uri, List<int>> _byteCache = {};
 
-  SourceFileProvider(this.byteReader);
+  SourceFileProvider(this.byteReader, {this.disableByteCache = true});
 
   Future<api.Input<List<int>>> readBytesFromUri(
       Uri resourceUri, api.InputKind inputKind) {
@@ -56,9 +58,11 @@ abstract class SourceFileProvider implements api.CompilerInput {
     if (!resourceUri.isAbsolute) {
       resourceUri = cwd.resolveUri(resourceUri);
     }
-    // Do not register URIs with schemes other than 'file' as these are internal
-    // libraries that we cannot read later.
+
     registerUri(resourceUri);
+    if (!disableByteCache) {
+      _byteCache[resourceUri] = source;
+    }
   }
 
   /// Registers the URI and returns true if the URI is new.
@@ -115,6 +119,11 @@ abstract class SourceFileProvider implements api.CompilerInput {
     if (!resourceUri.isAbsolute) {
       resourceUri = cwd.resolveUri(resourceUri);
     }
+
+    if (_byteCache.containsKey(resourceUri)) {
+      return _sourceToFile(
+          resourceUri, _byteCache[resourceUri]!, api.InputKind.UTF8);
+    }
     return resourceUri.isScheme('file')
         ? _readFromFileSync(resourceUri, api.InputKind.UTF8)
         : null;
@@ -152,8 +161,8 @@ Uint8List readAll(String filename, {bool zeroTerminated = true}) {
 
 class CompilerSourceFileProvider extends SourceFileProvider {
   CompilerSourceFileProvider(
-      {SourceFileByteReader byteReader =
-          const MemoryCopySourceFileByteReader()})
+      {SourceFileByteReader byteReader = const MemoryCopySourceFileByteReader(),
+      super.disableByteCache})
       : super(byteReader);
 
   @override
@@ -541,7 +550,8 @@ class _BinaryOutputSinkWrapper extends api.BinaryOutputSink {
 class BazelInputProvider extends SourceFileProvider {
   final List<Uri> dirs;
 
-  BazelInputProvider(List<String> searchPaths, super.byteReader)
+  BazelInputProvider(List<String> searchPaths, super.byteReader,
+      {super.disableByteCache})
       : dirs = searchPaths.map(_resolve).toList();
 
   static Uri _resolve(String path) => Uri.base.resolve(path);
@@ -588,7 +598,8 @@ class MultiRootInputProvider extends SourceFileProvider {
   final List<Uri> roots;
   final String markerScheme;
 
-  MultiRootInputProvider(this.markerScheme, this.roots, super.byteReader);
+  MultiRootInputProvider(this.markerScheme, this.roots, super.byteReader,
+      {super.disableByteCache});
 
   @override
   Future<api.Input<List<int>>> readFromUri(Uri uri,