Implement JsKernelToElementMap through KernelToElementMapBase

pkg/compiler/lib/src/kernel/env.dart

Index: pkg/compiler/lib/src/kernel/env.dart
diff --git a/pkg/compiler/lib/src/kernel/env.dart b/pkg/compiler/lib/src/kernel/env.dart
index 74ec5ba43b58f626a36da6dce42f71f749a1f618..c0185a45434ac3be54037621559f494c51983413 100644
--- a/pkg/compiler/lib/src/kernel/env.dart
+++ b/pkg/compiler/lib/src/kernel/env.dart
@@ -23,15 +23,15 @@ import 'element_map_mixins.dart';
 
 /// Environment for fast lookup of program libraries.
 class ProgramEnv {
-  final Set<ir.Program> programs = new Set<ir.Program>();
+  final Set<ir.Program> _programs = new Set<ir.Program>();
 
   Map<Uri, LibraryEnv> _libraryMap;
 
   /// TODO(johnniwinther): Handle arbitrary load order if needed.
-  ir.Member get mainMethod => programs.first?.mainMethod;
+  ir.Member get mainMethod => _programs.first?.mainMethod;
 
   void addProgram(ir.Program program) {
-    if (programs.add(program)) {
+    if (_programs.add(program)) {
       if (_libraryMap != null) {
         _addLibraries(program);
       }
@@ -47,7 +47,7 @@ class ProgramEnv {
   void _ensureLibraryMap() {
     if (_libraryMap == null) {
       _libraryMap = <Uri, LibraryEnv>{};
-      for (ir.Program program in programs) {
+      for (ir.Program program in _programs) {
         _addLibraries(program);
       }
     }
@@ -70,6 +70,14 @@ class ProgramEnv {
     _ensureLibraryMap();
     return _libraryMap.length;
   }
+
+  /// Copy all [LibraryEnv]s from [other] to this environment.
+  void copyFrom(ProgramEnv other) {
+    if (other._libraryMap == null) return;
+    _programs.addAll(other._programs);
+    _libraryMap ??= <Uri, LibraryEnv>{};
+    _libraryMap.addAll(other._libraryMap);
+  }
 }
 
 /// Environment for fast lookup of library classes and members.