[dart2js] Prepare for locals.dart migration

`element_map_interfaces.dart` contains temporary interfaces that exist for the duration of the nnbd migration.

`element_map_migrated.dart` contains 'real' code from `element_map.dart` that has been migrated. This will get re-combined with the `element_map.dart` once it is all migrated.

Change-Id: I55cc414de471adbe2d63c0e226558d5b4773d409
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249744
Reviewed-by: Mayank Patke <[email protected]>
Commit-Queue: Nate Biggs <[email protected]>

Author: natebiggs

pkg/compiler/lib/src/closure.dart

@@ -12,7 +12,12 @@ import 'js_model/element_map.dart';
 import 'serialization/serialization_interfaces.dart';
 
 export 'closure_migrated.dart'
-    show BoxLocal, JSEntity, PrivatelyNamedJSEntity, ThisLocal;
+    show
+        BoxLocal,
+        JSEntity,
+        PrivatelyNamedJSEntity,
+        ThisLocal,
+        TypeVariableLocal;
 
 /// Class that provides information for how closures are rewritten/represented
 /// to preserve Dart semantics when compiled to JavaScript. Given a particular
@@ -320,31 +325,3 @@ class ClosureRepresentationInfo extends ScopeInfo {
   // just confusing.
   bool get isClosure => false;
 }
-
-/// A type variable as a local variable.
-class TypeVariableLocal implements Local {
-  final TypeVariableEntity typeVariable;
-
-  TypeVariableLocal(this.typeVariable);
-
-  @override
-  String get name => typeVariable.name;
-
-  @override
-  int get hashCode => typeVariable.hashCode;
-
-  @override
-  bool operator ==(other) {
-    if (other is! TypeVariableLocal) return false;
-    return typeVariable == other.typeVariable;
-  }
-
-  @override
-  String toString() {
-    StringBuffer sb = StringBuffer();
-    sb.write('type_variable_local(');
-    sb.write(typeVariable);
-    sb.write(')');
-    return sb.toString();
-  }
-}

pkg/compiler/lib/src/closure_migrated.dart

@@ -4,6 +4,34 @@
 
 import 'elements/entities.dart';
 
+/// A type variable as a local variable.
+class TypeVariableLocal implements Local {
+  final TypeVariableEntity typeVariable;
+
+  TypeVariableLocal(this.typeVariable);
+
+  @override
+  String? get name => typeVariable.name;
+
+  @override
+  int get hashCode => typeVariable.hashCode;
+
+  @override
+  bool operator ==(other) {
+    if (other is! TypeVariableLocal) return false;
+    return typeVariable == other.typeVariable;
+  }
+
+  @override
+  String toString() {
+    StringBuffer sb = StringBuffer();
+    sb.write('type_variable_local(');
+    sb.write(typeVariable);
+    sb.write(')');
+    return sb.toString();
+  }
+}
+
 /// A local variable used encode the direct (uncaptured) references to [this].
 class ThisLocal extends Local {
   final ClassEntity enclosingClass;