Redo "Work-around for missing mixin-application charOffset"

pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
index d8df4f86fc88338d1476241da82eef9b8281e558..181d22ca1c575294f7a71091beb59270dd3313c4 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
@@ -290,6 +290,10 @@ class KernelLibraryBuilder
       int unresolvedCount = 0;
       Map<String, TypeBuilder> freeTypes = <String, TypeBuilder>{};
 
+      // TODO(30316): Use correct locations of mixin applications
+      // (e.g. identifiers for mixed-in classes).
+      if (charOffset == -1) charOffset = type.charOffset;
+
       if (name == null || type.mixins.length != 1) {
         TypeBuilder last = type.mixins.last;
 
@@ -419,8 +423,8 @@ class KernelLibraryBuilder
         checkArguments(mixin);
         supertype = applyMixin(supertype, mixin, signature,
             isSyntheticMixinImplementation: true,
-            typeVariables:
-                new List<TypeVariableBuilder>.from(variables.values));
+            typeVariables: new List<TypeVariableBuilder>.from(variables.values),
+            charOffset: charOffset);
       }
       KernelNamedTypeBuilder mixin = type.mixins.last;