[cfe] Check name conflicts using fragments

Change-Id: I7ffe54bdce190de5b31303fd794947aba3d3b1c3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/393102
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>

pkg/front_end/lib/src/fragment/extension.dart

@@ -36,6 +36,8 @@ class ExtensionFragment extends DeclarationFragment implements Fragment {
             ? new FixedExtensionName(name)
             : new UnnamedExtensionName();
 
+  bool get isUnnamed => extensionName.isUnnamedExtension;
+
   @override
   SourceExtensionBuilder get builder {
     assert(_builder != null, "Builder has not been computed for $this.");

pkg/front_end/lib/src/fragment/field.dart

@@ -34,6 +34,19 @@ class FieldFragment implements Fragment {
       : _initializerToken = initializerToken,
         _constInitializerToken = constInitializerToken;
 
+  // Coverage-ignore(suite): Not run.
+  bool get hasSetter {
+    if (modifiers.isFinal) {
+      if (modifiers.isLate) {
+        return !modifiers.hasInitializer;
+      } else {
+        return false;
+      }
+    } else {
+      return true;
+    }
+  }
+
   Token? get initializerToken {
     Token? result = _initializerToken;
     // Ensure that we don't hold onto the token.

pkg/front_end/lib/src/fragment/util.dart

@@ -15,14 +15,23 @@ class ConstructorName {
   final int? nameOffset;
 
   /// The name of the constructor including the enclosing declaration name.
+  ///
+  /// For unnamed constructors the full name is normalized to be the class name,
+  /// regardless of whether the constructor was declared with 'new'.
+  ///
+  /// For invalid constructor names, the full name is normalized to use the
+  /// class name as prefix, regardless of whether the declaration did so.
+  ///
+  /// This means that not in all cases is the text pointed to by
+  /// [fullNameOffset] and [fullNameLength] the same as the [fullName].
   final String fullName;
 
-  /// The offset at which [fullName] occurs.
+  /// The offset at which the full name occurs.
   ///
   /// This is used in messages to put the `^` at the start of the [fullName].
   final int fullNameOffset;
 
-  /// The number of characters of [fullName] that occurs at [fullNameOffset].
+  /// The number of characters of full name that occurs at [fullNameOffset].
   ///
   /// This is used in messages to put the right amount of `^` under the name.
   final int fullNameLength;
@@ -32,5 +41,6 @@ class ConstructorName {
       required this.nameOffset,
       required this.fullName,
       required this.fullNameOffset,
-      required this.fullNameLength});
+      required this.fullNameLength})
+      : assert(name != 'new');
 }

pkg/front_end/lib/src/source/source_builder_factory.dart

@@ -1871,13 +1871,20 @@ class BuilderFactoryImpl implements BuilderFactory, BuilderFactoryResult {
       suffix = identifier.name;
       suffixOffset = identifier.nameOffset;
       charOffset = qualifier.nameOffset;
-      fullName = '${prefix}.${suffix}';
+      String prefixAndSuffix = '${prefix}.${suffix}';
       fullNameOffset = qualifier.nameOffset;
       // If the there is no space between the prefix and suffix we use the full
       // length as the name length. Otherwise the full name has no length.
       fullNameLength = fullNameOffset + prefix.length + 1 == suffixOffset
-          ? fullName.length
+          ? prefixAndSuffix.length
           : noLength;
+      if (suffix == "new") {
+        // Normalize `Class.new` to `Class`.
+        suffix = '';
+        fullName = className;
+      } else {
+        fullName = '$className.$suffix';
+      }
     } else {
       prefix = identifier.name;
       suffix = null;
@@ -1887,16 +1894,17 @@ class BuilderFactoryImpl implements BuilderFactory, BuilderFactoryResult {
       fullNameOffset = identifier.nameOffset;
       fullNameLength = prefix.length;
     }
-    if (libraryFeatures.constructorTearoffs.isEnabled) {
-      suffix = suffix == "new" ? "" : suffix;
-    }
+
     if (prefix == className) {
       return new ConstructorName(
           name: suffix ?? '',
           nameOffset: suffixOffset,
           fullName: fullName,
           fullNameOffset: fullNameOffset,
           fullNameLength: fullNameLength);
+    } else if (suffix == null) {
+      // Normalize `foo` in `Class` to `Class.foo`.
+      fullName = '$className.$prefix';
     }
     if (suffix == null && !isFactory) {
       // This method is called because the syntax indicated that this is a
@@ -1921,9 +1929,9 @@ class BuilderFactoryImpl implements BuilderFactory, BuilderFactoryResult {
     return new ConstructorName(
         name: suffix ?? prefix,
         nameOffset: suffixOffset,
-        fullName: suffix ?? prefix,
+        fullName: fullName,
         fullNameOffset: fullNameOffset,
-        fullNameLength: noLength);
+        fullNameLength: fullNameLength);
   }
 
   void _addNativeGetterFragment(GetterFragment fragment) {

pkg/front_end/lib/src/source/source_field_builder.dart

@@ -154,14 +154,8 @@ class SourceFieldBuilder extends SourceMemberBuilderImpl
       assert(lateIsSetSetterReference == null);
       assert(lateGetterReference == null);
       assert(lateSetterReference == null);
-      _fieldEncoding = new RepresentationFieldEncoding(
-          this,
-          name,
-          nameScheme,
-          fileUri,
-          nameOffset,
-          endOffset,
-          fieldGetterReference);
+      _fieldEncoding = new RepresentationFieldEncoding(this, name, nameScheme,
+          fileUri, nameOffset, endOffset, fieldGetterReference);
     } else if (isLate &&
         libraryBuilder.loader.target.backendTarget.isLateFieldLoweringEnabled(
             hasInitializer: hasInitializer,