[dart2js] More lenient source code handling for diagnostic reporting.

Should avoid exceptions as thrown in flutter/flutter#138326

Only register source code if there is content in the byte array so we know when we should try to read from file. And when trying to read contents of a file for diagnostics, if we fail to read the file then fail gracefully by using position information.

Change-Id: Ic7e2bdcba0b093a48735aa26670509354f953861
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/336480
Reviewed-by: Mayank Patke <[email protected]>
Commit-Queue: Nate Biggs <[email protected]>

Author: natebiggs

pkg/compiler/lib/src/source_file_provider.dart

@@ -63,7 +63,10 @@ abstract class SourceFileProvider implements api.CompilerInput {
     }
 
     registerUri(resourceUri);
-    if (!disableByteCache) {
+
+    // Source bytes can be empty when the dill has source content erased. In
+    // that case we should read the file contents from disk if we need them.
+    if (!disableByteCache && source.isNotEmpty) {
       _byteCache[resourceUri] = source;
     }
     sourceBytesFromDill += source.length;
@@ -95,6 +98,15 @@ abstract class SourceFileProvider implements api.CompilerInput {
     return _sourceToFile(Uri.parse(relativizeUri(uri)), source, inputKind);
   }
 
+  api.Input<List<int>>? _readFromFileSyncOrNull(
+      Uri uri, api.InputKind inputKind) {
+    try {
+      return _readFromFileSync(uri, inputKind);
+    } catch (_) {
+      return null;
+    }
+  }
+
   /// Read [resourceUri] directly as a UTF-8 file. If reading fails, `null` is
   /// returned.
   api.Input<List<int>>? readUtf8FromFileSyncForTesting(Uri resourceUri) {
@@ -129,7 +141,7 @@ abstract class SourceFileProvider implements api.CompilerInput {
           resourceUri, _byteCache[resourceUri]!, api.InputKind.UTF8);
     }
     return resourceUri.isScheme('file')
-        ? _readFromFileSync(resourceUri, api.InputKind.UTF8)
+        ? _readFromFileSyncOrNull(resourceUri, api.InputKind.UTF8)
         : null;
   }